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Your site is not providing the intermediate cert.
However you installed the cert, go back a review the files used.
One of them probably already includes the cert and intermediate content.
Simply change the file name used and restart your web service.
And you continue to give very little information…
Based on that little bit you just added, I would say that you are using the cert file and should probably be using a different file, or a combination of more than one file, along with the key file.
But which one(s)?
You haven’t provided enough information for anyone to be sure about anything.
That seems to be a tutorial for a tool called WP LetsEncrypt. Despite the name, that is not made by Let’s Encrypt and it’s important to note that most Let’s Encrypt users don’t use this tool and aren’t familiar with it.
The tutorial seems to be explaining how to install the resulting certificate in cPanel, but cPanel also has its own built-in Let’s Encrypt support which is probably easier to use and more reliable than using a tool like this (including because cPanel can handle certificate renewals for you automatically, which at least the workflow described in the video tutorial doesn’t seem to be capable of doing). Is there a reason that you need to use WP LetsEncrypt instead of cPanel’s built-in certificate functionality?