here is an example of what you were currently trying to do, but with correct syntax, make sure you dont put ";" after void tastmanager(){}; < when your not defining it you dont need to put it for implementation. also when you declare using namespace std, the compiler assumes the std:: prefix for everything so you dont need to put it.

Code:
#include <string>
#include <iostream>
#include <vector>
using namespace std;
void taskmanager();
	  

main ()
{ 
taskmanager();
cout << "\nBye\n\n\n\n\n\nAll Code Copyright Patrick \n\n\nAll Rights Reserved\n";

}

void taskmanager()
{
   int tasks[40];
   int input, num, i = 0;
   bool exit = false;   
   while (!exit)
   {
      cout << "\nHello \n This is your friendly task manager \n Select a command \n";
      cout << "1) Add a task \n2) Read next on list\n3)Quit\n";   

      cin >> num;
      switch (num)
      {
         case 1:
            cout<< "\nOK, enter your task:\n ";         
            cin >> tasks[i];
            i++;  
            break;
         case 2:
            if (i==0)
   	        {
               cout<<"\nSorry, no items to read\n";
	           input = 0;
  	        }
            else
            { 
               cout << "\nDO THIS:\n";
               cout << tasks[(i-1)];
               input = 0; 
            }
            break;
         case 3:
            exit = true;
            break;
         default: 
            cout << "Invalid Input";
      }
   }
}