It is broke in version 52. Supposed to be fixed in 52.1 to be released sometime next week.

Thanks for the quick response Airmail, I downloaded ver 52.1 and can now set up additional gmail accounts Much appreciated

I had the same problem, and the answer for me turned out to be a Gmail setting. It seems Gmail thinks thunderbird is "less secure" so I had to turn a toggle that allows less secure apps to access gmail and it worked fine https://support.google.com/accounts/answer/6010255

Use Oauth2 instead of Normal Password if you want the real answer. Then gmail does not complain about Less Secure.