I have never tried the OSX version of server, but if you’re getting a permission error, look at where manager is installed, who has access to run it and make sure you’re that user. You should be, but it’s weird.
Where is it installed?
Navigate to that folder in terminal and type: ls -al and check the user and group assigned to the manager folder and then the files within it.
Is that the same user you are logged in with? (it should be). I only run Mojave and so I have no idea about security issues and permissions for anything higher, it’s possible there is an issue there.
That is an issue I use to run across often in linux, just because something should be an executable, doesn’t mean it is so when it’s installed (un-archived), although not exactly permissions based, it kinda is. It is a permission to execute a file and although that’s not what I was looking for in my ls -al request, it would have been picked up.