Symptoms

After installing certificate you check the website and are presented with an SSL error:
The certificate is not trusted!
This means that you are not presented with a full chain of trust of the certificate.

Cause

SSL Certificate chain is incomplete. Some components are missed.
Use the full root and intermediate certificates when installing the certificate.

Resolution

Please use the full root and intermediate certificates when installing the certificate, these can be found in this knowledge base article.
You can check your installation on this website, we use it often as it has a lot of checks that can give you plenty of information about your server's security

Or use the tool from Sectigo, when you are using a Comodo (Sectigo) certificate.