Hi, Marissawith2s, and thanks for reaching out to us here in the Apple Support Communities. We understand that you're unable to use your email on your iPhone, after signing in using your Mac. We'll be glad to look into this with you further for a resolution.
From what you've described, it sounds like you may be attempting to add the account to your iPhone. If that is the case, you'll want to follow these exact steps to re-add the Yahoo email address to your iPhone: Add an email account to your iPhone, iPad, or iPod touch
If you've followed those, and are still having issues, please let us know exactly which steps you're performing on your iPhone to add the account, and the exact wording of any prompts or error messages you receive.
Thanks again for reaching out. Let us know if we can assist you further.
Cheers!