Save the command file and use it for debugging
When the plugin appears to be functioning but nothing appears to be happening to the metadata, the first question to ask is this - Have you remembered Metadata > Read Metadata?
The next question is whether the plugin has been installed in a "special" folder where the operating system may restrict Exiftool from running. For example, some people install the plugin in a folder inside the Lightroom program file which can mean that the dialog box displays but the OS prevents Exiftool running. I prefer to install plugins somewhere less-special like Documents or Dropbox.
In genral, if nothing appears to be happening, the problem may be running Exiftool or with the images, for example that they are read only. To help you find out what is going on, the plugin can save the Exiftool commands as a text file. You can use this to call Exiftool directly and run it in its interactive mode which displays messages about its activity and any problems it encountered:
In Windows, I think the easiest way is to change the log file's extension to .bat, and it will then run as a batch file.
On Mac I prefer to cut and paste into a Terminal window.