why does it say email is not available when I try to add my new email address?

A lack of details doesn't allow us to try to be more specific. Have you used this email address with Apple before? If yes, then from our reading dozens of posts from people wanting to re-use an email address that has been used in any capacity with Apple before you can't again. If no, then contact Apple. The people on this forum are just users like you. We only know what you tell us and our ability to provide assistance is limited to what we can post here. As I said, Apple only has small snippets about which email addresses can be used and when, mostly iCloud email addresses.

If you still have problems:

Click here --> https://support.apple.com/choose-country-region Select your country, then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.

or:

Contact Apple for support and service --> "See a list of Apple phone numbers around the world."

Click here --> https://support.apple.com/HT201232

Apple doesn't have a formal position statement on re-using email addresses but from reports on this forum it is generally understood if you have recently used an email address with Apple for another purpose such as a notification email address it cannot be re-used immediately for another purpose.

About your Apple ID email addresses (was "Manage your Apple ID primary, rescue, alternate, and notification email addresses") --> https://support.apple.com/HT201356