The option you are looking for only applies to POP email. IMAP is server based and you just remotely view the messages on the server. If you want a local copy you need to create a folder or folders under Local Folders and copy the message there before removing from the server.

I don't want a copy on the server, just the one that downloads. I want to know where I can uncheck the leave message on server

Please reread my answer.

So are you saying I must leave every message on the server? Can I change it to Pop3? I just got a new computer and I am 70 years old and the transfer from the old computer didn't work so I lost everything from the old computer and am going nuts, I really need help

In my answer I told you how to move messages into local folders and remove from the server. I am not sure what your issue to leaving them on the server is. Are you getting a mailbox full warning. As for changing from IMAP to POP, if your provider has both types of mail service, reinstall your account as Pop then move the messages out of the IMAP folders and into the new POP folders and then delete the IMAP account from Thunderbird.

I wish you were here to help me, I don't understand any of this.

Can I go to tools account settings and then just write pop3 over the IMAP or do I have to set up an entirely new account and delete the old one? I'm sorry if these are not intelligent questions but I have no idea what any of this means

I don't see a place for where to set up a new account, which is why I asked if I can just overlay the IMAP with POP3

I think I've got it figured out and would like to thank you for your help

Here is an article explaining the change from IMAP to POP. https://support.mozilla.org/en-US/kb/faq-changing-imap-pop

Thank you Airmail, I think I've finally got it. Thanks for all the help, I am a clutz about these things. I screwed up the data transfer and lost all the email contacts and everything from the old computer, but I've started over before.