I encountered this issue too, and there are two possible issues.
The first issue is if the prefix of your email has the word "app" in it -- that sends a security flag through the system and prevents you from continuing. You will have to create a new email where the prefix doesn't contain the word "app" in it.
The second is that if you are registering in association with an organization, that organization must have a website. Say it's something like xyz.edu.
Then, the email that you register with must have a domain of "xyz.edu" -- aka if your work email is "johndoe@gmail.com", then Apple Developer expects your email to be of the form "johndoe@xyz.edu" instead.
For further information, I would go to this website: Support - Apple Developer --> Contact Us (upper right hand corner) --> Membership and Account --> Program Enrollment --> Call Me. I received a call back within 2 minutes, and they were very helpful.