When you cannot access the yahoo mail account on the apple computer. It means there is something wrong with the yahoo mail settings. here are the guide to fix that.
1. Please make sure that there is no issues on the mac related to the internet.
2. If you can't access the yahoo mail using the web browser. you should clean up the browser and then retry to login your account.
3. Make sure that you are using the valid password during the login period.
4. Check the imap and pop settings on the apple mail application.
5. please reconfigure the account on the apple mail.
once you will make these changes on the account. your email will start working fine. in case, if you need more information, please feel free to visit: Can't log into yahoo mail account problems?