C little program help

C console programs always start in the main function, so a minimal C app would look like this:

#include <stdio.h>

int main()
    {
    printf("Hello World\n");
    return 0;
    }

All executable code except global variable definitions (with or without simple assignments) must be inside a function.
So your line:

int sum , n1 , n2; 

could be outside any function (but in this case is probably not a good idea as they dont; need to be global variables) but no other lines can be.
It's also a very good idea to indent your code - it makes it a lot more obvious what you expect to happen, and to match up brackets.
In addition, it's a good idea to always use curly brackets in if, else, and loops - particularly when you are just getting started.

So try like this:

#include <stdio.h> 
int main()
    {
    int sum , n1 , n2; 
    scanf("%d %d" , &n1 , &n2);
    if ((n1 == 10) || (n2 == 10))
        {
        printf("A number or their sum is 10");
        }
    else
        {
        sum = n1 + n2;
        if (sum == 10)
            {
            printf("A number or their sum is 10")
            }
        else 
            {
            printf("Numbers or their sum is not 10");
            }  
        }
    return 0;
    }

Quote:

__tester__.cpp:4:6: error: expected constructor, destructor, or type conversion before '(' token

Why .cpp? You are supposed to write a C (.c) program.
Try

#include <stdio.h>
  
int main()
{
  int n1 , n2;

  scanf("%d %d" , &n1 , &n2);

  if( (n1 == 10) || (n2 == 10) || ((n1+n2) == 10))
    printf("One number or their sum is 10\n");
  else
    printf("None of them or their sum is 10\n");

  return 0;
}