Invalid field access : set
"Invalid field access" is usually an indication for a null reference error. Perhaps you're doing something with a sprite that has already been destroyed?
Are other targets working? It seems likely they would have the same issue.
Compiling code from third chapter of tutorial "Groundwork"
It might be a bit early in the tutorial to do this, but whenever you run into trouble, you can compare your code to the finished version of the game here: