Translation:You can't read your texts underwater.
It's not nonsensical. It might even be true, given that "you" is a second person singular pronoun and could easily be referring to a person who cannot read texts underwater in a given circumstance.
There is a context. Teaching me "underwater" without a context would be a problem, but the context is clear.
not sure why "messages" is not accepted here. The next sentence in this exercise allows for "messages" and "texts" as both being correct. Je lis mes messages sur mon nouveau portable. vs You can't read your texts underwater. Any ideas as to why the acceptability (or lack thereof) for this word choice ? In the sentence "Paul envoie beaucoup de messages a Anna" also in this exercise, BOTH worlds are accepted as correct.