Don't have SSL certificate on my subdomain despite SSL/TLS encryption mode full mode is activated

I have a website that has an SSL certificate. However, the subdomain of this website is not encrypted and I don’t have the SSL certificate on that website.
Why is that? How can I apply the SSL certificate on the subdomain aswell?

Main website-


One more detail-
I only have one subdomain.

Loads fine for me on HTTPS -


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