How are you trying to access your Yahoo mail? I assume that it's not via web-based access (i.e. going to the Yahoo Mail website in Edge, Internet Explorer, Firefox, Chrome or whatever) but are you using the Windows 10 Mail App, Windows Live Mail, Thunderbird or something else? Or are you using the dedicated Yahoo Mail App for Windows 10?
If it's the Windows 10 Mail app, have you added your Yahoo Mail account to it? If not, see here (but, obviously, select 'Yahoo! Mail' instead of 'Google'):
http://www.groovypost.com/howto/add-gmail-email-windows-10-mail-app/