It seems to be a near-consensus among the regular contributors to this site that Grammarly and similar software are more confusing than helpful, and that it is best to simply ignore them. Even if one wanted to be more charitable to those who write and use such software, one would have to point out that it has never been (or should have never been) intended as a substitute for, but only as supplement to, learning about writing from human teachers. The software does not, and cannot, create authoritative rules for writing, but only get the users to think about, and apply, what they have learnt about writing elsewhere. Thus if such software generates a message that it appears that something is wrong, the crucial word there is appears. The point of the message is not that one must change what one has written, but that one should pause and think about whether one's wording is right, in light of what one knows about the matter. If one is confident that it is right, one should leave it as it is.
In this case, the relevant 'rules' would be the ones about the order of adjectives, but they, as has already been pointed out by Mr. Ashworth in a comment, do not imply anything about the particular adjectives in question, which do not involve colour, shape, etc. Moreover, these 'rules' are not really rules, but more in the nature of stylistic advice, and they should always yield to the established practices of experienced writers in the relevant field. Thus if you can point to reputable sources that use constitutional federal republic (as the question does) that is sufficient to justify your using the same wording in your writing, and ignoring the Grammarly's warning.