If you are trying to send an EIP-1559 transaction but the current network does not support it, there are several steps you can take to try and resolve the issue. First, try changing your MetaMask network back and forth, as this may fix the issue. If that does not work, you can try disconnecting and reconnecting your wallet manually, changing networks back and forth, hard refreshing the page (CTRL+F5) and clearing the cache, updating your MetaMask/wallet to make sure it is compatible with EIP-1559, resetting your MetaMask account (which will not log you out), or importing your wallet to MetaMask. If none of these steps work, it is possible that the network you are using does not support EIP-1559, and you will need to switch networks to a network that does support it in order to send the transaction.