Trouble of adding subdomain certificate to space object storage

Hi, I have a domain in cloudflare and created a Origin Certificate of it. Then I add the certificate to space object storage, there always has a trouble when resolving the certificate.

We encountered an error while setting up your custom subdomain due to an invalid certificate. We can help create your custom subdomain by generating a certificate for you. Alternatively, consider selecting a different custom subdomain.

I created the certificate from cloudflare->SSL/TSL->Origin Server->Origin Certificates.

And also set the Current encryption mode to Full (strict).

So How can I find out the problem?

And I also want to know How many subdomains can add to a burkets?

Thanks!