I see. Well, according to the article I linked, it can take up to 24 hours for the changes to be updated everywhere and recognized by the system, so check in every now and then to see if it has been updated.
Even if the client area doesn’t show the CNAME record as set up, you can just hit the Add Domain button and the request will be sent to the control panel.
The control panel has it’s own domain validation, so sometimes you can add a domain when the client area says it’s not ready yet, or not add the domain even though the client area says it should work.
Both the client area and the control panel are subject to DNS caching. But due to how DNS caching works, different systems can show different results.