Please enter a valid domain
Is there any typo like “whitespace” while copy-paste or writing the domain?
Maybe you are trying to add an domain with unsupported tld?
Maybe it could be related to an internal issue in Cloudflare?
Currently I do not see any incidents or issues regarding it at Cloudflare Status page:
The domain name that needs to be added to. Shop cannot be added. The domain name that you try to add to. Com cannot be added
Can you screenshot the page when this error occurs?