You cannot create an new AppleID account using an email address already registered and associated with an existing AppleID account.
As described, you appear to be using two different AppleID accounts - one for your iPhone - and another for your iPad. You are perhaps unwittingly attempting to create a third [new] AppleID account using the email address already used to identify the existing AppleID account being used with your iPhone.
If so - and you are currently using a different AppleID with your iPad - you just need to sign-out from iCloud, App Store, FaceTime and other services on your iPad. When you have signed-out, restart your iPad - and then simply sign-in using the AppleID already being used with your iPhone.
Sign out from FaceTime
Settings > FaceTime > [AppleID] > Sign Out
Sign out from the App Store
Settings > [Your Name / AppleID] > Media & Purchases > Sign Out
Sign out from iCloud
Settings > [Your Name / AppleID] > Sign Out
Restart your iPad
Restart your iPad - Apple Support
After restarting your iPad, sign-in to your iPad using your existing iPhone AppleID credentials.