Please follow the instructions in our official spine-godot Runtimes guide. Everything else will lead to errors on your end, and questions directed at us which we can't answer, because you are not using the setup we documented.
You can find the info here:
You must follow this exactly, or else you will pull in the official Godot C# .dlls, which do not contain the Spine APIs.
I've justed ported the GDScript
example-v4 Godot project to Godot + C#. You can see the full setup there (and see how spine-godot is used with C#):