The information on voting on tag synonyms previously stated:
Users with a total answer score of 5 or more, can vote for tag synonyms.
The previous question Clarify the documentation for tag synonyms added the following text to clarify what "total answer score of 5 or more" meant:
(total upvotes minus total downvotes)
However, it appears that the actual calculation of "total answer score" is total upvotes minus total downvotes for the current tag. This is an important distinction as it means users with less than 2,500 reputation cannot vote on synonyms for new tags in their areas of interest, if they have not posted much under that particular new tag yet, but the message explaining why they cannot vote is arguably misleading.
Case in point: as a relatively new Stack Overflow user, my total upvotes minus total down votes is 57. My reputation is 879. Based on the criteria as stated, it would appear I could vote for the tag synonym [objc-arc] for the tag [automatic-ref-counting], but I cannot—with the message above.