why can't I delete my email?

Sorry, but that is just not enough information in order for us to be able to assist you with a potential solution.

• Writing an effective Apple Support Communities question - Apple Support Communities

For example:

• Are you using the Apple Mail app or accessing your email via a web browser?
• Does this issue occur for one specific email account or for all of them?
• What have you tried, so far yourself, to resolve this?

Start by updating the computer to most recent version released Dec 13, 2021 >> Monterey 12.1

Make Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS before the update is applied.

Try Rebuilding the Mailbox. Go to Mail>Messages and Rebuild is at the bottom of the list of actions.

What happens when you right click on a message and select Delete?