Please fill out the fields below so we can help you better. Note: you must provide your domain name to get help. Domain names for issued certificates are all made public in Certificate Transparency logs (e.g. crt.sh | example.com), so withholding your domain name here does not increase secrecy, but only makes it harder for us to provide help.
It surely produced more output than that. But that aside, the error is pretty clear, isn't it? Let's Encrypt needs to connect to your site via HTTP in order to validate that you own it, and it can't connect. Neither can I. Neither can letsdebug.net: Let's Debug
You'll need to figure out why that is (the most likely reason, as the error message tells you, is a firewall issue) and fix it before you'll be able to get a cert this way.
I see you switched to proxying your domain name at Cloudflare. That's perfectly fine of course but Cloudflare CDN cannot reach your home server either. Cloudflare issues a 523 error so see the first link below for debug help.
You also might consider using the Cloudflare Origin CA Cert. You use this instead of a Let's Encrypt cert on your home server to encrypt the traffic between Cloudflare's CDN Edge and your Origin Server (your home server). See its link below