If you shared your site name then people could look at your certificate and have more information to help you like whether DNS changes have propagated.
My best guess is that whatever certificate you got is expired. Can you say how or where you got it? The instructions you linked to don’t describe how to get a certificate.
The simplest solution would be to turn off cloudflare and rebuild your site with Let’s Encrypt.
You can turn off force_https like this:
cd /var/discourse
./launcher enter app
rails c
SiteSetting.force_https=false
exit
exit
But most browsers (like yours) will remember that the site is supposed to be https and will try very hard not to connect. Or, more elegantly: