Transaction Status: 0 / not confirmed, not in mempool Trying to send BTC, but fail. The transaction is not in mempool.

What to do with these and how to solve the problem?

I already returned BTC canceled the transaction
Is the software you're using Bitcoin Core?

Then you need to make your software forget your unconfirmed transaction that is not in the mempool, so that you can make it again with a higher fee.

When launching your Bitcoin Core, run it with -zapwallettxes. This'll make your wallet forget the unconfirmed transactions relevant to your wallet. You can learn how to add that on Andrew Chow's blog.

Then the balance in your wallet should become the balance before you made the payment.

I've learned this from here, but the other two methods aren't applicable as you need to change your transaction to increase the fee.