gmail Imap accounts use OAuth2 as Authentication Method and therefore require cookies to be enabled.

So check this to see if it is the reason you are having a problem. In Thunderbird

• Menu app icon > Options/Preferences > Privacy & Security
• Under 'Web Content' section
• Select checkbox 'Accept cookies from sites'

I'm assuming you trying to set up an imap mail account?

You say " it still says it can't gain access" I'm assuming you are creating a New > Existing mail account you enter name, email address and password and select the checkbox to remember password Then click on 'Continue' Thunderbird then checks for the configuration data

Does Thunderbird locate the server settings? OR do you get the error message at this point and what is the error message word for word ?

Have you just installed Thunderbird ? If yes, then exit Thunderbird if it is running and check your Firewall because Thunderbird must be in the list as an allowed program.