Corrupted macOS disk permissions may prevent mInstaller from properly finalizing the installation.
To fix them, please try the following:
Close all applications running in the background (especially Motion and FCP), otherwise the files and folders that are in use will be omitted,
Open mInstaller and uninstall the affected product,
Go to mInstaller Settings via the Cogwheel Menu, [Repair] your Disk Permissions and [Clear] mInstaller's preferences (do not log back in onto the app just yet),
Open the Disk Utility (you'll find it in your Applications > Utilities folder), select your disk in the menu on the left side of the app's screen, and press "First Aid",
Select the partition below your disk, press First Aid again and, once the process has been finalized, close the Disk Utility,
Next, reboot your Mac, log onto mInstaller, install the previously uninstalled plugin and check if it helped.
If not, please re-install the plugin via the PKG installer that can be downloaded from your My Products page (hover your cursor over the preview miniature until the "Download" button appears).