SSL not working with subdomain

Please I want to setup my sub-domain to work with SSL too as my main domain via Cpanel partner settings?
I have been trying it for long but I cant really understand why it’s still not working.

As the question, seems like it is not related to Cloudflare, rather to your cPanel and AutoSSL option?

You can achieve this, getting free Universal SSL or purchasing a Dedicated SSL on Cloudflare, by adding your domain to Cloudflare.

Have you tried to generate your SSL certificate for main and sub-domain via cPanel AutoSSL option (if it is available to you?).
Is the domain/Website already added to the Cloudflare?