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Did all of the steps in that tutorial come back OK, with no errors?
Is it possible that you have a firewall either on the VPS or in your hosting plan setup that prevents inbound connections from the Internet on port 443? In that case everything might already be working properly but the firewall could be preventing us from seeing that.
From the warning at the top of the guide you followed:
The instructions in this guide install Let’s Encrypt and add certificates manually (…)
Did you by any chance install the certificates? Because your apachectl -S command doesn't show any HTTPS VirtualHosts.
On another note: the guide is terrible and very old (4 years is eternity in IT land..). It uses git to clone the whole certbot repository for just the certbot-auto script (using the very, VERY ancient name letsencrypt...). It also uses git pull to update the client? I've never seen that. Didn't the certbot-auto script update itself back in 2016?
The letsdebug site is only testing things that could stop certificates from being issued, but we already know that that's not a problem because your certificate was issued correctly.
Something that the guide is not helping with and that @Osiris noticed is that the guide uses certonly, a mode where Certbot does not install your certificates in your web server application (which requires changing /etc/apache2). You would have to do that yourself with this guide—otherwise your Apache doesn't know that the certificates exist!
You would probably have a better experience using the instructions at https://certbot.eff.org/ because, as @Osiris noted, this guide is horribly out of date in various ways. I originally wrote some of the features that the guide refers to, and I can confirm that it's not using the up-to-date names for them!
Most notably, if you use certbot --apache instead of certbot certonly --standalone, Certbot should take care of installing the certificate for you by modifying your Apache configuration files!
OK, I'll give it a try. Was just following instruction given to me.. What do I do with my current certificate and key? Just delete everything or ignore?
Getting and installing a certificate are two very different things! With that specific Linode guide, you're required to manually edit your webservers configuration files to add a HTTPS virtualhost and configure it to use the certificates you've gotten earlier.
Well, I'm not familiair with Linode, but most of the stuff of the guide is hopelessly obsolete.
Just keep them, certbot will recognise it and use it if it can.