It's likely you are using Authentication Method : Oauth2 So any password you enter is not the expected password.

Settings > Privacy & Security Under Passwords Click on 'Saved Paswords' Click on 'Show Passwords

You may get asked to enter a password in a 'Windows Security' window That means - use the same password you use when you start computer and logon to account.

Then remove all the gmail passwords - which may be mailbox, imap smtp or oauth. click on Close

Restart Thunderbird

Google will open a window asking for password so you can say allow Thunderbird access to server. Enter the normal password you use when logging onto webmail

That should get a new oauth token in Thunderbird.

I used the password I use to log into google webmail. The message I get is a pop-up box asking 'Mozilla Thunderbird Email wants to access your Google Account' the top of the box says Enter credentials for ###@### on imap.gmail.com. It only asked for the password once and there is no entries in the saved passwords box