why can't I use my email as an apple id

Without more information, no one here would be able to answer that question.

What happens when you try to use your email? What is the exact wording of the error message you're getting? Are you setting up a new Apple ID? Or changing an existing one?

How to create a new Apple ID - Apple Support

Change your Apple ID - Apple Support

Normally, if your e-mail address was associated with an ID in any way, for example as a Reachable at or Rescue e-mail address, then it can't be reused with another Apple ID for approximately one year. I have also seen posts that stated Apple told them there was a 30 day waiting period before the address could be used again. You will need to get another e-mail address to use as your Apple ID or you can try waiting 30 days to see if that works.