You must use the MPLAB® Harmony Configurator (MHC) plugin version bundled with the particular version of MPLAB Harmony you are using. You cannot assume the latest MHC plugin version (found by selecting the 'Available Plugins' tab) will be compatible with your Harmony version.
The MHC plugin (*.nbm file) comes with the MPLAB Harmony download and can be found in the following folder: <install-dir>/utilities/mhc. The default install directory is C:/Microchip/harmony/<version number>.
If you are using multiple versions of Harmony on one computer, you will have to replace (uninstall and reinstall) one MHC version with another. Another alternative is to install different MPLAB X IDE versions on the same computer and install a different MHC plugin version in each IDE.
Visit the MHC plugin installation guide for more details.