I had the exact same issue from a fresh install of Kubuntu 20.04 (Plasma 5.18.5, Kmail 5.13.3).

Removing the account and configuring it again solved the issue for me as well.

Extra note :
I configured 3 IMAP accounts before noticing the disabled "Move to trash" button. The faulty account was on the same provider than another one working fine. I can't remember which one I configured first. But I remember that no email was displayed at first in the INBOX folder on the faulty account (whereas the other folders on the faulty account had emails and the INBOX folder on the other account on the same provider also had emails). I had to tick the server side subscription checkbox from the configuration to start displaying emails, but could not delete them before removing and configuring again the account.