Sometimes staring at code for hours can make you gloss over the simple items, and all it needs is a fresh set of eyes.
The type declaration for the variable area is missing. The Compiler thinks that you have declared a type "area" and have chosen to not give it a name.
Also, your DisplayResults function doesn't return any values, so you should change the return type to void.
Try this:
void DisplayResults(int area) {
cout << "\n" << "The area of the rectangle is: " << area << "\n";
}