it means compiler don’t know memory size of your struct, because it most likely don’t have full deceleration of it to know proper size. In .cpp file you should have include both header file with deceleration of struct before the class it self.
You might also have name conflict as in UE4 there already struct with FItemData:
That might be the case as you not include that you struct header in class header yet it see the struct. You should forward refrence in class header like this:
TArray<struct FStructName> MyArray;
then by including struct before class header file in cpp it will prevent potential circle refrence