Hi Gogo,
actually, your question has been asked quite a few times now ( within different thread titles ).
Pls. use for example the Plesk documentation as reference:
If you experience any issues/errors/problems, pls. don't forget your add your current used operating system, your current used Plesk version ( incl. #MU ) and try to explain "
step-by-step", what you did to reach your goal, so that people willing to help have something to investigate and are then able to suggest solutions and work-arounds, to solve your described issues/errors/problems.
In addition, pls. be informed about the ( free of charge )
Plesk Let's Encrypt extension (
=> Let's Encrypt - Plesk Extensions ), with which you are able to install a ( completely free of charge certificate ) for your
Plesk Control Panel.
You might as well be interested, that with
Plesk Onyx, the goal might be reached much faster, as their are a lot of improvements made for securing the Plesk Control Panel / Webmail and your mail - server.