There could be several reasons.
- The endpoint isn't working. To debug this, test with a different endpoint (the free tier from most providers is enough for debugging).
- The Optimism network is down. Check here to see if this is the case.
- There's a problem with the wallet. Try a different wallet that supports Optimism.
- You do not have enough ETH on Optimism for the transaction. The transaction cost on Optimism is divided between an L2 execution fee and an L1 security fee. Some wallets only check you have enough to cover the L2 execution fee, which is usually the smaller of the two.