Kindly, I’d suggest you to fix your SSL certificate and make sure to have a valid installed at your origin host/server before moving to the Cloudflare.
Before moving to Cloudflare, was your Website working over HTTPS connection with a valid SSL certificate?
Is your sub-domain hostname like sub.domain.com or www.sub.domain.com?
Kindly, before doing anything at Cloudfalre settings, you could determine if you have a valid SSL certificate installed at the origin host/server by your web hosting provider or your own VPS/dedicated server following the steps from below:
Use the “Pause Cloudflare on Site” option from the Overview tab for your domain at dash.cloudflare.com .
The link is in the lower right corner of that page.
Give it five minutes to take effect, then make sure site is working as expected with HTTPS.
Check with your hosting provider / cPanel AutoSSL / ACME.sh / Certbot / Let’s Encrypt or some other and renew it accordingly.
Only then should you un-pause Cloudflare and double-check your SSL/TLS setting to make sure it’s Full (Strict).
Here is a way to re-check if you correctly setup the SSL for your domain with Cloudflare: