From Kant's Lectures on Ethics[Infield, tr., Hackett (Indianapolis: 1980)]:
The statement 'fac bonum et omitte malum' cannot, however, be a basic principle of moral obligation, because the good can be good in diverse ways according to the end chosen, the statement being thus an axiom of skill or of prudence, whereas for it to embody a moral principle it would have to imply that which is good for moral action. (p. 25)
This particular version of the first precept of natural law is from Wolff (although in context Kant attributes it to Baumgarten); it is in line with the traditional Thomistic view although not exactly the same. However, the difference is not particularly important for my purposes, because Kant is in fact right here. This is not a problem for traditional natural law theory because traditional natural law theory is structured on this very point.
'Good is to be done and sought, bad is to be avoided' or 'do good and avoid bad' are not direct statements of moral obligation. There are many reasons for this, but the basic point is that these are general principles of practical reason itself. They cover all practical fields about which we can rationally deliberate, not just morality, and that includes, as Kant correctly recognizes, matters of skill and of prudence.
Kant will go on to say that it is tautological, which, depending on what precisely you take that mean, is also arguably right. His idea is that the command 'fac bonum' here just boils down to 'it is good that actions happen that are good'. One can quibble about the precise formulation, but there is a legitimate truth here, one that is often misunderstood even by people wishing to understand natural law: the most fundamental precepts of natural law are supposed to divide rational from irrational behavior, and therefore violating them is supposed to be irrational, just like violating the principle of noncontradiction. But this, too, is not a problem for natural law theory, since natural law theory is deliberately built like this.
So both the objections are true as far as they go. However, that they are not serious problems for natural law theory in general can be seen by looking at the natural law theory of Thomas Aquinas. In Aquinas's account, 'Good is to be done and sought, bad is to be avoided' is explicitly the practical reason counterpart of the principle of noncontradiction. Just as all theoretical reasoning is held, in one way or another, to the standard of the principle of noncontradiction, and just as any reasoning that violates the the principle of noncontradiction is irrational, so too with practical reason and the first principle of practical reason. (Indeed, although it's not an essential point to make here, I think there is a very good argument that Aquinas thinks they are in a sense the same, given the coextensiveness of good and being.) Thus all practical reasoning whatsoever is governed by the first principle of practical reason, 'Good is to be done and sought, bad is to be avoided'. This is true of your decision to tie your shoe, of your choice of what to eat for lunch, of where to go on vacation, of, in short, everything in your practical life.
It also applies to moral obligation. But how does the obligation arise? In Aquinas what we call 'obligation' he calls 'law' -- indeed on any natural law theory, the latter is the more proper term, with 'obligation' being a metaphor for what law does. Aquinas famously defines a law according to its four Aristotelian causes:
A law is a particular ordering of reason to common good by one who is caretaker for what is common, promulgated.
So if we look at the first principle of practical reason just on its own, what is missing for it to be an obligation? It is obviously an ordering of reason; indeed, there is a sense in which it is the ordering of reason, the most fundamental one. Because it can be known by any rational person, it is promulgated, that is, it is in principle possible for those who are supposed to follow it, to follow it, which is the point of promulgation. What is missing is common good. The first principle of practical reason on its own is an ordering to good, but it is not restricted to common good. Once we do restrict it, that is apply it to that kind of good which is common good, we get the caretaker clause immediately; on Aquinas's account the natural caretaker for any common good is the whole body of rational beings whose common good it is.
There are lots of different kinds of common goods. To get morality in the full sense, we would have to apply the principle to that common good that is the most general common good we human beings definitely share. And that is the common good of the entire human race. That common good involves a lot of different subsidiary goods, but they concern, at least among other things, the goods we share that are relevant to our individual physical survival, the survival of the human race, and our survival as rational social beings. Given this, the first principle of practical reason becomes the first precept of natural law, the fundamental obligation.
And it is very fundamental, and very deliberately so. The point of identifying the first precept is not to identify directly what the right thing in any particular case is. You can't do that without reasoning about the goods involved in the particular case. (This is something Kant himself smuggles in, unacknowledged, in his notion of a 'maxim'.) The first precept provides a standard that has to be met by every action and all practical reasoning that touches on matters of common good. In general, natural law theorists don't think it is the only standard; it's just the most general one. In your explicit reasoning it might not even show up, just as you don't normally have to introduce the principle of noncontradiction as a premise in an argument about theoretical subjects. But it is what makes your reasoning about what you ought to do coherent at all.
So in a natural law account, being moral is acting reasonably with regard to matters important to us all. Natural law theorists do not make the sharp distinction between the pragmatic and the moral that Kant does; they are the same kind of things, just differing as to the kinds of goods in view. And once natural law is recognized, then all other kinds of law and obligation have to conform to it -- if they violate it they violate both reason and common good, so fail at two of the four requirements for a law.
What is more, since the most fundamental precepts of natural law are part of what it is to be rational in the first place, all moral reasoning whatsoever involves them, in the same sense that all reasoning involves the principle of noncontradiction. This includes arguments that never explicitly talk about natural law. All ethical arguments are natural law arguments; the only question is whether they are good arguments. This is also something that is often forgotten even by people who want to affirm natural law theory: natural law theory is not a theory of arguments that explicitly mention 'natural law', but a theory of all moral reasoning whatsoever.
Other natural law theories are not always so clean and clear-cut as Aquinas's, so some of these points are a bit more complicated in those cases than they are when we are talking about the Thomistic version. But in pretty much every case, its true that the first precepts of natural law are specified versions of more general practical principles, and they are concerned with rational consistency in our actions, whatever goods we may consider, so that what we ought to do has to consider the goods involved, and not just the abstract formal principle. So, again, Kant's two objections are right; they are, in fact, deliberate elements of natural law theory, and for the natural law theorist they are not bugs but features. Kant wouldn't be satisfied by that, of course, but that's only to be expected.