The problme is as the error says:
-s;
Only assignment, call, increment, decrement, and new object expressions can be used as a statement
"-s" is none of those - you would have to assign the value somewhere in order for it to be acceptable.
space x = -s;
Console.Write("x:");
x.display();
BTW: It is very bad practice to affect the current item in a negation operator: it is not clear that that would occur. Think about it: does it happen when you do this:
int i = 17;
int j = i + -i;
No, j is assigned zero. If the negation operator worked your way, then it would be assigned -34, probably...
Use the commented out version to return a new instance instead.