Can I create a subdomain on a Shopify store with an SSL?

Yes, it is possible to have a subdomain on Shopify with an SSL. Shopify gives users the provision to purchase and use a number of domains under the primary domains that they are using. And an SSL certificate, i.e. a Secure Sockets Layer is a provision allowing the authentication, encryption, and advanced security of a site.

So if you are looking to get a subdomain on any Shopify store, your SSL Certificate will be applicable on it since it is connected to Shopify itself. Either way, your primary domain will be the domain where the customers and other viewers will be directed to.

Shopify allows its users to have only 1 SSL per server. And on this 1 SSL, you can include 10 domains/subdomains on your Shopify store. Since you want a WordPress server, you'll need 2 SSL certificates—one for the Shopify server and the other for the WordPress server. You can use Cloudflare to seamlessly put a WordPress subdomain on your Shopify store.

While it is possible to have sub domains on Shopify, the platform only allows for one SSL per server. You could choose to have up to ten domains on that SSL. In this instance you will probably need to get another SSL certificate for the Wordpress server. Use a service such as Cloudflare to help you with this.

Only domains hosted or connected to Shopify will have SSL certificates from Shopify. Because you will be putting a WordPress on a sub-domain, you will need an additional SSL certificate from that domain host. Do put the WordPress domain on your Shopify store, you can use the service, Cloudfare.