From this error code, you are trying to make a transaction using inputs which are not valid according to blockchain.com.

This could be due to a number of things. I do not think blockchain.com allows you to spend unconfirmed transactions, so if you have incoming coins you are trying to spend which are yet to be confirmed, then you could get this error. It could also be that you have outgoing transactions which are yet to be confirmed, and your blockchain.com wallet is trying to include those inputs in your new transaction, which it is then rejecting as a double spend. Are there any incoming or outgoing transactions in your wallet which are still unconfirmed?

Given that the mempool remains very full and backed up, then this is unlikely to resolve on its own. I would export your seed phrase to Electrum, following the instructions from BlackHatCoiner above to select a BIP39 wallet, and then attempt to make your transaction again using Electrum.