How do I use my email address as my Apple ID?

If you want to change your ID, have you followed the steps in this article? Make sure you turn of Find My Phone before signing out of the old ID. Once signed out and Find My Phone is off, then sign in with the new ID, then turn Find My Phone back on.

What to do after you change your Apple ID or password - Apple Support.  *. 

Enter the Apple ID that you used on the iPad in 2018. That is still an Apple ID unless you deleted the ID or deactivated the ID.

Are you trying to make a new ID when you get the error message or are you entering the email address to sign in?