May I ask what SSL option have you got selected under the SSL/TLS tab at Cloudflare dashboard for your domain ( Flexible, Full, Full Strict … )?
Before moving to Cloudflare, was your Website working over HTTPS connection?
Can you double-check if your web hosting provider issued an SSL certificate which is still a valid one for your domain(s)?
Before doing anything at Cloudflare 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.
Check with your hosting provider / cPanel AutoSSL / ACME.sh / Certbot / Let’s Encrypt or some other and renew it accordingly.
Make sure site is working as expected without any errors via HTTPS.
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: