Yahoo wants you to. use their mail app.
Increasingly Yahoo is ‘encouraging’ users to stop using third party apps and use theirs instead (they want that ad revenue they get if you use their app)
If you do have the yahoo mail app installed and it isn’t working, you need to take it up with them. They may have a glitch they will fix. You can search the reviews for the yahoo mail app (you will need to change the sort from most helpful to most recent) and see if others are also having issues.
It may also be something behind the scenes that yahoo will quietly fix and it will suddenly just start to work.
