The access denied error means that something is preventing your Windows account from writing the certificate data.
If you go to C:\ProgramData\GPSoftware\Directory Opus and delete dopus.cert then try it again, that may resolve things.
On a normal Windows 10 setup, you should see a UAC prompt when installing the certificate. Does that happen OK?
If it still isn't working, making a Process Monitor log of what happens when you install the certificate may reveal what's going wrong. Please zip and send the log (or a cloud storage link to it) via email or private message if you make one.