You need to call the function GetName on an instance of a Cat. Cat is an abstract idea, and clearly you cannot get the name of Cat, but you can get the name of a Cat. The syntax you are using only works for static funtions (which don't require an associated instance of the class).

Code:
int main()
{
  char name[20];
  Cat felix;
  felix.GetName(name);
  return 0;
}