If you get this error:
you are most probable on a new version like Unity2020.03.11 and above. It seems that the DLL we are including in game4autiomation is in newer versions already included in Unity.
To solve this please select in your project the Newtonsoft.Json.dll:
And delete the DLL.
Now apply again the Game4Automation standard settings, because the standard installer script might failed before due to the error:
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