Why can't I change email address on my Apple ID

Based on your screenshot you need to choose a different address, see this Apple article: Change your Apple ID

Change your Apple ID - Apple Support (SG)

👉 If you can't change your Apple ID

If you try to change your Apple ID to an @icloud.com email address that was created within the last 30 days, you will be asked to try again later.

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. 

Then see this Apple article: What to do after you change your Apple ID or password

What to do after you change your Apple ID or password - Apple Support

See also -> Manage and use your Apple ID

Manage and use your Apple ID – Apple Support (UK)

Click here for more support and assistance.

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.