As far as I can tell, I am on the newer by mail platform as opposed to yahoo but I'm unable to login to the new app.
Try to log in using web page and see if it says YAHOO in top left
the new app only works for customers on new BT mail
Going by your previous posts I suspect you are on the "old" BTMail system so it will not work for you and going by the reviews that the app gets it doesn't work for anybody else regardless which system they are on.
At the moment this app will only work for users who were on the BTYahoo email system and have been migrated to the "new" BTMail since December 2019.
It will not work for users on the "old" BTMail system ie have not been migrated or for those still on the BTYahoo system.