C code error: I am not sure why I get this error

Quote:

I tried adding long long for NUM, but that didn't help. GetLongLong() is supposed to be a built-in function.

/*
 * Reads a line of text from standard input and returns an equivalent
 * long long in the range [-2^63 + 1, 2^63 - 2], if possible; if text
 * does not represent such a long long, user is prompted to retry.
 * Leading and trailing whitespace is ignored.  For simplicity, overflow
 * is not detected.  If line can't be read, returns LLONG_MAX.
 */

long long
GetLongLong(void)
{
    // try to get a long long from user
    while (true)
    {
        // get line of text, returning LLONG_MAX on failure
        string line = GetString();
        if (line == NULL)
            return LLONG_MAX;

        // return a long long if only a long long (possibly with
        // leading and/or trailing whitespace) was provided
        long long n; char c;
        if (sscanf(line, " %lld %c", &n, &c) == 1)
        {
            free(line);
            return n;
        }
        else
        {
            free(line);
            printf("Retry: ");
        }
    }
}

long long GetLongLong()
{
    char *endptr;
    string line = GetString();
    long long num = strtoll(line, &endptr, 10);
    // Conversion stops at first char that is not a digit.
    // May inspect *endptr to detect this.
    // Zero if not a number.
    // LLONG_MAX or LLONG_MIN if out of range (errno is set to ERANGE).
    free(line);
    return num;
}