Apologies if this question has been addressed somewhere on these forums. I have searched but couldn't find anything. I have recently upgraded to 10.14.3. I now find that I cannot install any software which does not appear on the App Store. I get the error message "Ossystem error 99999". Prior to this I have visited Sys Prefs>Security and Privacy and unlocked the lock and clicked "Open Anyway". I then get the 99999 error. I have searched online for the past few days but only one fix could be found which didn't work - sudo cp /var/db/.SystemPolicy-default /var/db/SystemPolicy (PW and Restart). I was able to use Win 7 via VMWare Fusion to use the windows version of the SW I wanted to use but this is not a practical workaround long term.
The SW in question: makeMKV, DVD Extractor, XLD and some others I cannot now remember. However, Tag Editor from the App Store worked fine.
There appears to be no other issues with Mojave. All works OK. I have run all available system checks and repair routines including fsck -f in Single User Mode. I have also cleared PRAM.
I am an experienced Mac user but this problem has me stumped.