Hi, MichaelZakaria.
We appreciate you taking the time to post in the Apple Support Communities. We understand that you’re getting prompted to enter your email password for either Gmail, iCloud, or Yahoo on your iPhone, and when you add it, it says it’s already added on your iPhone. We’d like to see if we can help.
In your screenshot, we can see that you’re attempting to enter your iCloud account when you get the message you described. Your iCloud email account is likely managed in the iCloud settings on your device. If you go to Settings > [your name] > iCloud > Mail, is this toggled on? This may be why you’re seeing the message for iCloud. This article will explain:
If iCloud Mail isn't working - Apple Support
For your other accounts, try the steps outlined in these articles:
If you can't receive email on your iPhone, iPad, or iPod touch - Apple Support
If you can’t send email on your iPhone, iPad, or iPod touch - Apple Support
Warmest Regards.