Deal with error lead by multiple Newtonsoft Json DLL file.

1. Back up the project to prevent any crash.

2. Search for “Newtonsoft.Json.dll” and delete them.

   

3. Go to Edit > Project Settings > Player Settings > Other Settings > Configuration, uncheck the option Assembly Version Validation.

   

4. Install Newtonsoft Json from Package Manager.

   

5. Refresh the project.