I can't up-vote, I have 15 reputation, the requirement. Can anyone tell me why I can't up-vote? I have tried on up-voting a Terraria question, is that my problem? Also this was my first up-vote attempt.
Quickly glancing at your profile on StackExchange shows you have 11 reputation, not 15. If you had 15 rep at one point you may have needed to wait a day for the StackExchange server to register the change and lift the restriction.