Unable to send funds out of brave wallet - #10 by brian explains what the checksum means.
It can be ignored, it is a UI only check to warn you that you are sending to an address that can’t be verified by Brave.
It’s not the issue that you are experiencing.
Do your transactions have a transaction hash when it ends in error and when it is submitted?
You’re just trying to send from the Brave interface or a dapp?