I am trying to resign my coaches, (we are in the playoffs but I am eliminated) and I give them the same salary they are currently getting but it tells me I am -$2 in cap.
Not sure what's going on here

Might as wait and resign them at free agency 1 unless it's the last year of their contract. Signing them during the playoffs will still expose them to being poached during next season's free agency.

Never tried to offer the same salary, I always offer a token raise, even if it's just a dollar.

Yes, always do +$1 and you're safe. 100% never an issue.