The three most common issues for this error are:
1. Your SSL hasn't been verified by our SSL vendors yet. The verification process typically takes roughly 10 minutes during business hours for our paid SSL certificates, or up to 24 hours when ordering a free Universal SSL certificate.
2. Something has gone wrong with the issuance of the SSL cert, or our vendors have put an additional security check in place before issuing the certificate.
3. Your DNS Records on the Cloudflare dashboard are 'grey-clouded' and you are bypassing our proxy. For Cloudflare to present our certificates you need to be passing through an 'orange-clouded' DNS record.
4. Cloudflare's SSL, if you are not getting errors on the root domain (yoursite.com) and www (www.yoursite.com) over https://, can only go one level on subdomains. This can be fixed with Dedicated Certificate with Custom Hostnames through Cloudflare.
blog.yourdomain.com would be fine (one level)
travel.blog.yourdomain.com would not work (two levels)
Please read this KB article for more details.