There is a bug in Thunderbird where if a hard connection error occurs the perfectly good password (and any password you enter when prompted) fails. The workaround is to delete the saved password using tools -> options --> security -> passwords -> saved passwords, exit (since the deleted password is still in memory), and restart.
A gmail IMAP account can use either a password or a OAuth2 token as the authentication method. Configuring OAuth2 is usually easier to troubleshoot as you mainly need to worry just about clicking on "Settings -> Forwarding and POP/IMAP ->Enable IMAP" in the Gmail webmail's settings, and checking that cookies are enabled for google.com in Thunderbird. Cookies are normally enabled by default (Tools -> Options -> Privacy -> Accept cookies from web site is checked, and "Keep until" is set to "until they expire").
http://kb.mozillazine.org/Gmail
By Sky do you mean Sky Yahoo? According to
https://www.sky.com/help/articles/ge...sky-yahoo-mail it requires you to create a app password.and use it instead of the normal password in Thunderbird. That is very similar to using a Gmail account with Two step verification enabled (it too requires you to use a app password in a email client instead of your accounts password)