I wrote del anno instead of dell'anno (listening test). The correction said pay attention to the 'gender'.
Surely anno is masculine? which would mean that del is correct?
OR should it be dello because it starts with a vowel hence dell'anno?
Seems like you are getting a generic error message that's misleading. The better message would be "pay attention to spelling." With most masculine singular nouns, it's del, but if it's a masculine singular noun starting with a vowel, it's dell'.