The page for the create tag synonyms privileges, it states that:
How do I propose or vote on tag synonyms?
Users with more than 2500 reputation and a total answer score of 5 or more on the tag, can suggest tag synonyms. Users with a total answer score (total upvotes minus total downvotes) of 5 or more on the tag, can vote for tag synonyms.
But this privilege is awarded at 1250 reputation.