“This email is already in use with another Apple ID?”

Unfortunately Apple does not have a clear position statement about when you can re-use email address you have used with Apple before.

From: Change your Apple ID - Apple Support

"If you see a message that the email address you want to use isn't available or is already in use, check that you or a family member aren't already using that email address with a different Apple ID. If a family member is using that email address, remove it from their account then try to use it."

Although this next section discusses email address re-use in the context of deleting an account, it supports observations frequently posted on this forum about not being able to re-use an email address as an Apple ID once used previously as an Apple ID:

From: How to delete your Apple ID account - Apple Support

"Can I use the primary email address for my deleted Apple ID account to create a new Apple ID or with another existing Apple ID account?

No, once your Apple ID is deleted, you can't use the email address that you used with it to create a new Apple ID. In addition, you can't use that email address with another existing Apple ID except as a rescue [notification] email. Also, if your primary Apple ID email address is with a Custom Email Domain, it can never be reused to create an Apple ID after your Apple ID is deleted, even if the domain ownership changes."

If you still have questions or problems:

Refer to this document for ways to contact Apple --> Choose your country or region - Official Apple Support

Select your country (also look for "other" regions), 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 by telephone --> Contact Apple for support and service - Apple Support

Also worth noting: Add and manage email aliases for iCloud Mail on iCloud.com - Apple Support - "Note: You can’t delete or turn off a primary iCloud Mail address. You can’t use an alias to create a separate Apple ID, and an email alias can’t be converted to a primary iCloud Mail address."