What it means
Before asking you to sign, the wallet simulated the transaction against current chain state, and the simulation reverted, so no gas estimate exists. This is a preview of a failure, shown before you pay for it.
How to fix it
Treat it as a stop sign, not an obstacle. Do not set a manual gas limit to force it through: the transaction will very likely revert on-chain and cost real gas. Find the underlying revert first, most often a missing approval, wrong network, or insufficient balance.