There are a few things to consider if you are having difficulties with the installation of a certificate on plesk.
You will need to take into account what operating system you are using for your server.
On an apache based server if is vitally important that the intermediate and root certificates are uploaded in the correct order. If these are uploaded in the incorrect order, or with any files missing, the installation will fail and you will receive untrusted warnings when visiting the website. The order these certificates need to be uploaded, how and where they should be uploaded is clearly explained in the sets of instructions that we provide for each of our product ranges, please insure that you are following the instructions exactly.
If you are using a Microsoft Windows server the installation of the intermediate and root certificates is slightly different.
Firstly you will need to install the site certificate and key file as per the installation instructions for plesk.
You will then need to follow the instructions under the title "InstantSSL Certificate Installation: Microsoft IIS Root and Intermediate Certificate installation" under the relevant product section for the SSL certificate product that you have purchased.