I do agree that saying "you don't have the required score" is pretty useless, and of course someone who doesn't know what the required score is will be confused because you didn't tell them. There's no need to include the actual name of the tag in the notification, though. You're on the tag's synonym page, you should know what tag we're referring to.
I say just replace "the required score" with "a score of 5 or more" so it reads:
You do not have a score of 5 or more on this tag to vote for this tag synonym
Quick and simple and ya... Because I'm sure no one is actually reading that paragraph of text... It's a paragraph... and as far as I'm aware, the score of 5 thing doesn't change across different sites, only the reputation requirement does. So there's really no reason that can't be included in a non-dynamic error message.