The lock indicates the account uses a secure connection. It would have been there since you add the account to Thunderbird. What is keeping you from adding a folder

Thanks but answer is incomplete. What is keeping me from adding the folder is what?

That is the question for you. What are you doing? What is not working?

Ah, sorry. I go to create new folder and enter the name of the new folder. When I close the command the folder does not appear in the list.

Is this an IMAP account? Go to File-Subscribe Is the new folder there? If so subscribe to it.

That solved the problem. Many Thanks!