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It produced this output: Successfully ssl installation. But my site has a mixed content from http and https URL's
My web server is (include version): Ubuntu 20.04 x64
The operating system my web server runs on is (include version): WordPress on Ubuntu 20.04 x64
My hosting provider, if applicable, is: VULTR
I can login to a root shell on my machine (yes or no, or I don't know): YES
I'm using a control panel to manage my site (no, or provide the name and version of the control panel): NO
The version of my client is (e.g. output of `certbot --version` or `certbot-auto --version` if you're using Certbot): certbot - auto --version
Could you please elaborate more of your issue? If you already found out mixed content is the issue, we can't help you, as mixed content is a problem of the websites content and not the certificate.
Mixed content means some of your site is loading stuff (images, css or js) using http, instead of via https. You usually have to update this in WordPress, you will be able to google how to do that.
Actually, yes, the problem isn't the certificate but the mixed content and how to fix it. A plugin shows me the effective URL's and img. But I don't know how to fix it.
Thanks anyway.
This page on your site is using https:// in the image URL but it's using using an IP address
e.g. https://70.34.223.236/wp-content/uploads/2021/10/cropped-image_2021_05_09T21_44_13_194Z-145x94-1-e1635683934621.png
So you either need to edit the content individually and fix the URLs (use the proper website name and use https everywhere) or find a plugin that will do the search/replace for you. I've done this years ago in WordPress and back then the easiest thing to do was search and replace strings in the database itself (posts etc), nowadays I'd imagine there is a plugin that would do it.
Googling for it with wordpress fix http to https, this article suggests as plugin called Better Search and Replace, I've no idea if thats good/safe or not but it appears credible: How to Fix Common SSL Issues in WordPress (Beginner's Guide)