When trying to build nipkg files on my build system, I got a rather confusing error message from my System Exec node (with command cmd /s /c "nipkg pack "%s" "%s"" where the %s are replaced with the appropriate paths using Format into String).
Error -125089: A plugin returned one or more errors while creating a package: \\?\C:\...\data
Additional error information:
Error -125057: Cannot create the specified package. Missing a required attribute key-value pair 'Conflicts: ni-package-manager' in the control file.
the path given is the data subdirectory of the first argument - the source directory for the pkg.
On closer inspection, this has seemingly nothing to do with the 'Conflicts' attribute (which is not required).
The control file had an extra line break after the "Depends" attribute, and removing this allowed the build to succeed.
So if you hit this error (and find this via Google etc) then check that you don't have extra trailing lines being inserted to the control file via substitutions etc.