New Thunderbird account not connecting to gmail account

[Phil]

Thank you for reading this and I feel a little bit silly asking such a 
basic question but I've come to a dead end.

I've set up a gmail account for a relative. E-mails can be sent and 
received from that account. The problem is my relative finds web mail 
confusing and difficult to use so I though I'd set up a Thunderbird 
account . I've used Thunderbird and gmail for well over twenty years 
without any problems at all but setting up a new account has me stumped.

Thunderbird automatically sets up the account and authorisation but 
Google continually asks for the password but refuses the connection 
claiming the password is incorrect. Of course the password is correct 
otherwise web mail wouldn't work.

Some research shows that Google refuses the connection if the client is 
aware of the password. I had initially entered the password into 
Thunderbird's set up dialogue box and thought that maybe the problem. 
I've tried to create a new Thunderbird account without the password but 
Google goes through the same routine again refusing the connection. I've 
also tried Evolution and Claws-mail.

What silly mistake have I made?

-- 
Regards,
Phil