Unable to sign into my Apple Account on iPhone

1. Check credentials: Make sure you’re entering the correct Apple account and password.
2. Try signing in at account.apple.com on your iPhone’s browser.
3. Update iOS: Make sure your iPhone is running the latest iOS version.
4. Check your internet connection.
5. Restart your iPhone.
6. Reset password: If the issue persists, reset your Apple account password
7. Contact Apple Support if none of the above works.

Sign in to your Apple Account on the web or in your device settings - Apple Support

If you forgot your Apple Account password - Apple Support

If you recently changed either of those, make sure it is updated everywhere where those might be found. After changing email address I found I had problems signing in because Apple was automatically filling in with my old email address when I needed to sign in. Check all your Contacts entries for yourself.