Not sure if it qualifies as functionally illiterate, but the most prominent thing I could think of was when he had trouble reading the Constitution:
https://www.gq-magazine.co.uk/...
Don't shoot the messenger; this was reported on multiple sources. You can find the others if you don't believe this outlet.
Not evidence of illiteracy, but Trump is famous for, unlike all other presidents, not being able to digest the daily briefings in writing, instead getting them verbally and in pictures. I think there's no way the same information density can be conveyed without the text, and being able to process the full information is a basic job requirement for a President.
https://www.washingtonpost.com...
From what I gather, even if not "functionally illiterate", his reading level is well below what it should be for such an important job. I would even propose that being able to read The Constitution should be a job requirement in itself.
Other evidence, none of which is conclusive in isolation, but demonstrates a general pattern of lack of vocabulary and comprehension:
-Conflating "political asylum" with "insane asylum"
-Conflating "incursion" and "excursion"
-Not knowing what "currency devaluation" means
-not knowing who pays tariffs.
Even if you [the generic "you", not you personally.] support this administration, you shouldn't give them a free pass on doing a crappy job. In fact, that would be incentive to hold them to a higher standard. "It's OK that he's incompetent because I like him." is not good for any of us, supporter or not.