[EDIT]
Maybe the problem is that CloudFlare HTTPS is using only one cert and it is not supported by your browser???:
Certificate #1: EC 256 bits (SHA256withECDSA)
CloudFlare DNS isn't a problem, but you appear to be also using the CloudFlare CDN, which is.
When you have the orange cloud enabled for a domain in CloudFlare control panel, they run an intermediary server between your origin server and the Internet that caches content and serves as a web application firewall. This obscures your origin server so the Let's Encrypt certificate you installed will not appear.
You could turn off the orange cloud in CloudFlare, in which case they would not provide these services for you, but your original server would appear with your Let's Encrypt certificate.
Or, you could enable SSL in the CloudFlare control panel, and they'll provide a certificate for you. You still need a certificate to secure the connection between CloudFlare and your server. CloudFlare provides their Origin CA for this purpose. But, if you enabled Let's Encrypt successfully with no errors in the Plesk control panel you should be able to enable "Full SSL" with no problems as is. This will use your Let's Encrypt certificate to secure the connection between your origin and CloudFlare and a CloudFlare certificate between them and the Internet.
EDIT: @rg305 points out you already have SSL enabled in CloudFlare. If it is set to "Full", your Let's Encrypt certificate (or perhaps some other one) is working. (If it is not set to Full, your connections between your origin server and CloudFlare may not be secure.)
If you were just trying to force https for all your visitors, you can do that with a page rule: