What it means
An older contract tried to subtract a larger number from a smaller one, which SafeMath blocks. In practice this is how pre-0.8 Solidity contracts say insufficient balance or insufficient allowance: the subtraction that failed was balance minus amount.
How to fix it
Treat it as a balance or allowance problem: check you hold enough of the token and that the contract is approved for the amount. The wording is just an older contract's way of saying the same thing.