Q: help

Add new Mailboxes plus "+" sign

You can find the "+" sign at the top in the sidebar, next to Mailboxes.

It is hidden by default.

Move the mouse pointer over to the top and hover "Mailboxes".

or Mail > Mailbox > New Mailbox

Try:

1. restart
2. use Disk Utility to repair your HD. Using Disk Utility to verify or repair disks - Apple Support
3. try a test user account https://support.apple.com/en-us/HT204443
4. try a safe boot https://support.apple.com/en-us/HT201262
5. reinstall your current OS over the current installation
6. restore your HD from the backup you made with Time Machine prior to the upgrade Use Time Machine to back up or restore your Mac - Apple Support
7. if you have any of the following installed (you shouldn't), they may be the problem:
   1. third-party "antivirus" software
   2. "cleaners" or other shi* software like CleanMyMac or MacKeeper
8. If you have "antivirus" software installed, uninstall it and restart
9. If you have "cleaners" installed, erase your HD and start over from scratch