Apple ID issue

See this document to see what forms of email address are acceptable. Change your Apple ID  https://support.apple.com/en-us/HT202667 Read these two sections very carefully to see what is allowed in terms of form: If your Apple ID is a third-party email address and How to change your Apple ID if it ends with @icloud.com, @me.com, or @mac.com.m/en-us/HT201356

I realize that you may not be trying to change your Apple ID.

What are you trying to do when you see the error message?