There are many ways to delete an email and I'm none the wiser on what you are doing, so I'll offer a list of various methods. Please note: do not attempt to delete or indeed do anything whilst Thunderbird is compacting - wait until it has finished.

1. Select email in list so it is open and you can read it in Message Pane - click on the 'Delete' button in the Message Header area.
2. Right click on email in Message List and click on the Trash bin icon located top right
3. Select email in list and use the Menu Bar toolbar - select 'Edit' > click on 'Delete Message'
4. Select email in list and use Menu bar via keyboard 'Alt E' and then 'D'
5. Select email in list and press the 'Delete' key on keyboard.
6. In Table View - Enable the 'Delete' column header so you see a grey trash bin icon adjacent to each email in list - click on the relevant bin icon to delete message.

It would be useful to know where you are trying to delete an email.

Are you talking about a folder and email in a POP or IMAP account or Local Folders account ? Are you talking about email in a normal folder OR a 'Unified' folder which you may see if using 'View' > 'Folders' > 'Unified folders'? Is the email in a default folder or a folder you created ? Are you having a problem with just one particular email or is this a problem with deleting any email ?

a problem with just one particular email. Can't get rid of it.

I have a lot of emails after being away from my computer for 6 days. I was deleting them, but part of the way when i clicked [delete] the email would not be shown and then 2 seconds later it was back in m Inbox, Why is this happening?