I get an error when using blockinput

Hi, I am modifying a small application in c ++ that allows me to take control of my mouse and my keyboard, the code is as follows:

#include <windows.h>
#include <conio.h>
#include <stdio.h>


int main()
{
    HINSTANCE hDLL = LoadLibraryW (L"user32.dll");
    if (hDLL == NULL)
    {
        fprintf (stderr, "Failed to load user32.dll, error code %d\n", GetLastError());
        return 1;
    }


    typedef BOOL (WINAPI *BLOCKINPUT)(BOOL);
    BLOCKINPUT pBlockInput;
    pBlockInput = (BLOCKINPUT)GetProcAddress (hDLL, "BlockInput");
    if (pBlockInput == NULL)
    {
        fprintf (stderr, "Failed to import BlockInput, error code %d\n", GetLastError());
        FreeLibrary(hDLL);
        return 1;
    }

    else
    {
        BOOL res = pBlockInput (TRUE);
        if (!res) {
           fprintf (stderr, "BlockInput failed, error code %d\n", GetLastError());
        }
        Sleep (20);
    }



    FreeLibrary(hDLL);
    _getch();
}

The problem esque goes directly to the if it seems that the res variable is always false so I can not do what I'm trying:

What I want is that basically with blockinput for a while nobody can use my keyboard and mouse.