I assume you have followed the instructions as written on the below article:
Therefore, may I ask have you tried to contact Ghost support for it too?
Or, if recently changed nameservers to Cloudflare, usually it could take up to 24-48 hours due to the DNS propagation process.
Due to the fact that when using proxied mode with Cloudflare for that CNAME type DNS record, maybe their system checks and needs the Ghost (Pro) IP address in return, rather than a Cloudflare - which could be why it says "fail, unable to detect valid CNAME record for yourdomain.com"?
My best guess would be that you temporary setup the CNAME as you obviously did and then switch to instead, wait for few hours to apply the changes and therefore re-check until it confirms “success”.
Later on, when you get the SSL certificate from them and when it’s provisioned to you for your domain as stated from above articles for Ghost (Pro):
All Ghost(Pro) sites will automatically be provided with an SSL certificate by default, which simplifies the process and ensures you site is secure.
Therefore, you switch back to and make sure the SSLoption is set to Full (Strict) SSL at the SSL/TLS tab of Cloudflare dashboard for your domain: