I try to set duplex (two-sided) mode on my printer, but calling SetPrinter() is not working:

bool SetDuplexMode(HANDLE hPrinter, short duplexMode) {
    PRINTER_INFO_2* pInfo = NULL;
    DWORD dwNeeded = 0;
    GetPrinter(hPrinter, 2, NULL, 0, &dwNeeded);
    pInfo = (PRINTER_INFO_2*)malloc(dwNeeded);
    if (!GetPrinter(hPrinter, 2, (LPBYTE)pInfo, dwNeeded, &dwNeeded)) {
        free(pInfo);
        return false;
    }

    DEVMODE* pDevMode = pInfo->pDevMode;
    if (!pDevMode) {
        free(pInfo);
        return false;
    }

    pDevMode->dmFields |= DM_DUPLEX;
    pDevMode->dmDuplex = duplexMode;

    if (!SetPrinter(hPrinter, 2, (LPBYTE)pInfo, 0)) {
        std::wcout << L"SetPrinter not working" << L'n';
        free(pInfo);
        return false;
    }

    free(pInfo);
    return true;
}

I have this message :

SetPrinter not working