Trying to delete a subfolder in Thunderbird

[#1]

Somehow I created a subfolder I named 'emails to keep' in 'Trash' in Thunderbird IMAP.  It was a mistake, I don't even know how I managed to create it under 'Trash,' and I would like to delete it. When I try to move it or delete it a notice comes up: "The current operation on 'emails to keep' did not succeed. The mail server for account xxx 'my email address' @.plus.com responded 'mailbox does not exist or must be subscribed to.'
When I empty deleted emails in the trash folder everything is deleted apart from this subfolder. If anyone can suggest how I can delete this folder it will really be appreciated. If necessary how would I subscribe to it, please if that's how to delete it? I'm hoping it's nothing too complicated.
Thank you.

[#2]

Try moving it (using drag and drop) to be sub-folder of Inbox and then delete it?

[#3]

Thanks for replying, pwatson.  This is strange but, without doing anything else that  I'm aware of, the subfolder has disappeared.  I tried to delete it and tried to drag it to another location all with no success.  When I logged in a few minutes ago there was no sign of the subfolder 'emails to keep.' 
I hope it doesn't wind me up by reappearing.
Thank you again.