Folder creation requires a prefix to the name of the folder.
If you want a folder to store your sent messages, "sentmail," you would name it "INBOX.sentmail" for the new mailbox.
Because Webmail uses the IMAP protocol for an email server, you can create hierarchies of folders. These hierarchies keeps the Webmail interface on one screen.
Example: if you want to use "sentmail" to store your outgoing email but want your 2020 outgoing email to be "archived" and archived under "sentmail" then you can create "INBOX.sentmail.2020"
"INBOX.sentmail" must first exist and you will have to move the email items from "sentmail" to "sentmail.2020."