Is your account set up via IMAP? Make sure the sent folder in TB is mapped to the correct sent folder on the server

The reply is good as far as it goes, but prompts the question "How?"

What exactly do I need to do (as a non tech user of TB) to "Make sure the sent folder in TB is mapped to the correct sent folder on the server"?

I've searched though the various TB settings but couldn't find anything relevant. I'm probably looking in the wrong place, so I need to help please . . .

Thank you!

If Thunderbird doesn't have problems connecting to the IMAP server, this problem is typically caused by Thunderbird trying to save sent messages in a Sent folder that doesn't exist on the server and which Thunderbird won't try to create automatically either. You might solve that problem by manually creating the Sent folder yourself, but a folder for that purpose with a different name such as Sent Items may already exist on the server, in which case the right way to solve the problem is by telling Thunderbird to save sent messages in that folder instead.

Go to Account Settings > Copies & Folders. Under When sending messages, select Other. Then, amongst the options available there, look for a folder with a name that would indicate it's meant to save sent messages (e.g. Sent Items or similar). Choosing that folder should solve the problem.