Error Message
Custom SSL certificates are no longer supported for free subdomains. All free subdomains have SSL by default.
You literarily answered your own question. You can no longer manually create SSL certificates for a free subdomain on your own because these are handled automatically.
If you are trying to use a sub-subdomain then please understand that the support for these are entirely a mistake. The mistake is fixed and it’s no longer possible to obtain SSL for those domains.