In the original question, this was specifically about a "strawman fallacy" which I don't think is the right term. As modified and then answered by Geoffrey Thomas, I think this get's closer to a common objection to Kant that Kant scholars have had to grapple with.
In it's current form, the first paragraph states,
Kant argues that one should only act according to those principles which can be universalised. However human actions are often complex and motivated by multi-layered reasonings. They can rarely be distilled down to a simple one-sentence principle.
The first sentence is just simply what is meant by the universalization formulation of the Categorical Imperative. You can find it in the Groundwork and elsewhere.
The second and third sentences raise a claim that is somewhat debatable, because much hinges on how we describe human actions. Just to illustrate, Utilitarians are not committed to this since they think people act for pleasure (which need not be complex). Aristotelians and virtue ethicists tend to have a complex picture of action.
A second issue is whether we should understand actions in terms of motivations. The other option is to understand them in terms of reasons. The motivation picture is more Humean than Kantian (see Marcia Baron, Kantian Ethics Almost without Apology and Christine Korsgaard).
The relationship between these ideas is complex, but it seems like Kant believes it is possible to have a moral action which finds its origin in the will acting in accordance with reason. (This can be accompanied but not caused by a feeling or subjective motivation for Kant).
If this is what the objection is getting at, then it may be the case that the OP and Kant just have different ideas about what action is.
The second paragraph, however, highlights a real problem in Kant's philosophy. Kant never really gives us a clear explanation of what a "maxim" is or how it relates to one's action and how it becomes universal. Consequently, it is a real question whether we can pick the descriptions of the actions we undertake.
Allen Wood, Henry Allison, and Thomas Hill (inter alia) have all worked on this problem. My memory may be a little fuzzy here but the parts that I found convincing in this account are :
You cannot maxim shop -- it is not a question of picking a maxim that could explain your action and be universalizabe. It's a legitimate question of what you wanted to do when you acted. This seems pretty convincing to me, because maxims are not alibis; they are descriptions of the structure behind your actions.
Actions do admit some multiple descriptions the same action could be described in multiple ways. Kant admits the same in the quodlibetal questions that follow each section of the Metaphysics of Morals: Doctrine of Virtue.
Here, we have Kant saying that banter that includes untruths like "that's a splendid dress" or "of course you look fine" are not lies. Or similarly that willing that you die to protect others is not suicide. Or that killing in war is not murder.
The danger is that once you open the door to these sorts of re-descriptions, it's not clear how you justify some and not others. In my view, Kant does not have great resources for defending the quodlibetal questions. This is a central part of Hegel's critique of Kant (in Natural Law, again in Phenomenology of Spirit, again in Philosophy of Right) -- that "pure reason" alone is not adequate to create descriptions of real human situations and that we need societal knowledge to bridge the gap.
Many contemporary Kantians agree. Some of them argue that Kant was already saying this (Korsgaard, Rawls, Nancy Sherman, Thomas Hill, and Allen Wood -- who interestingly says this in conversion from the Hegelian position). Others admit Kant did not say this but don't care (Habermas).
To recap, there's two issues going on here:
- Kant's theory of action vs. a common contemporary one.
- The problem of "maxim shopping" with the partial response -- it's the reasons for which we actually undertake our actions plus the problem of description.