Pixyz Plugin for Unity requires lots of features from the Unity API and uses a lot of native code. Hence, it is not compatible with all desktop OS and versions of Unity.
You can get the compatibility table on the System Requirements page.
If you have a version of the Pixyz Plugin for Unity installed, you can also check the compatibility through the menu Pixyz > Check Compatibility
A popup will show up telling you if your version is compatible, not compatible or simply untested with your current setup.