I used a Hint power but I can't play the suggested word (AB2)

This is rare, but it does occasionally happen.

Alphabear 2 has multiple dictionaries, including some that provide word definitions (but let's pretend that they all comprise a single Definitions Dictionary) and one that determines whether a word is acceptable. The words in these dictionaries are not identical; there are some acceptable words for which we don't have definitions, and some unacceptable words for which we do have definitions. Unfortunately, for technical reasons we have to give certain kinds of hints with the Definitions Dictionary, even though (were it possible) we'd want to give them from the Acceptability Dictionary instead.

Getting a hint for a word you can't play should be a relatively rare occurrence since the Acceptability Dictionary contains more words than the Definitions Dictionary. However, it has been known to happen. In one known case, it happened because we decided to remove an otherwise-legitimate word from the Acceptability Dictionary (it was not a commonly known word and so we received a number of complaints when it was the only word on the board and people didn't know what to enter). Unfortunately, removing words from the Definitions Dictionary is harder, so we haven't done that and so the game will still give that word as a hint even though it is not submittable.