The full error message in the unity console is: Assets/Spine/spine-csharp/AnimationState.cs(910,25): error CS0066: `Spine.EventQueue.AnimationsChanged': event must be of a delegate type.
I think I may know the problem is. I have defined my own "Action" class in my games C# scripts, and this might be conflicting with a similarly named class in the Spine package. I'll try renaming my class and see if that fixes the issue.
Edit: That was the cause of the problem, and the issue is now fixed.