Hi @wizen,
The content of the “Complete C# Unity Game Developer 3D (2017-2019) Online Course” is archived. We do not support it anymore. A couple of months ago, Rick and Gary released their remastered Unity 3D course, which was created with Unity 2020. That’s the content we are currently supporting.
I do not know what you did exactly in your case but when importing the Standard Assets into your project, import only the things you need, not everything there is. If the package you need uses the new Input System and does not support the old one anymore, watch one or two tutorials on the new Input System on Youtube and try to replace the code for the old Input System with the code for the new Input System in your code.
Hopefully, this helped. 
See also:
- Forum User Guides : How to mark a topic as solved