This email address is not available. Choose a different address
This indicates that another AppleID has reserved your chosen email address as a secondary security, rescue, email alias or primary email for AppleID.
You have a couple options to fix this:

Go to each of your Apple ID you control and ensure no use of this email.
Contact Apple Support. They will generally never divulge the other account since they can’t tell you from someone trying to hack into your account so they may be able to help or they may tell you to choose another email or repeat step 1.