This email address is not available. Choose a different address.

See this Apple article: Change your Apple ID

Change your Apple ID - Apple Support (SG)

See also the following Apple articles:

Set up Family Sharing

Set up Family Sharing - Apple Support

What is Family Sharing?

What is Family Sharing? - Apple Support

Make sure that each family member has a unique Apple ID

Make sure that each family member has a unique Apple ID - Apple Support

Download family members’ purchases on iPhone

Download family members’ purchases on iPhone – Apple Support (UK)

Add a family member to your subscriptions from Apple

Add a family member to your subscriptions from Apple – Apple Support (UK)

If you are getting the error message in the title of your post, have you logged into your Apple ID to see if the address shows under Reachable At, as a Rescue email address, or as a Notification email address? If it is listed, you can remove/replace the email address.

Apple ID - Manage  

Apple hasn’t published any guidance for using an address again. Normally, if your e-mail address was associated with an ID in any way, for example as a Notification email address or a Rescue e-mail address, then it can't be reused as an  Apple ID email address 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 from somewhere like AOL or Yahoo to use as your Apple ID or you can try waiting 30 days to see if that works.