#include <iostream>
using namespace std ;

int calcArea( int , int ) ;
void display ( void ) ;
int main ()
{

int length , width , area ;

cout << "please enter the length and width" << endl ;
cin >> length >> width ;

area = calcArea ( length , width ) ;

display ( void ) ;


return 0 ;
}

int calcArea ( int l , int w )
{
return l * w ;
}

void display ( void )
{
return "display the area" ;

}

THERE IS SOMETHING WRONG ON THIS CODE. I CAN'T RUN THIS PROGRAMME.

When you call the display you should not write void inside the parenthesis.

Here you are returning constant character string from the function which has void as return type.
Either you do not return any thing (return; or //return "display the area" ; ) or change the return type (to char* display ( void ); in this example).