Why do I get error when ifstream is creating file with string read from keyboard?

I am doing exercises from Bjarne Stroustrup's book, Programming Principles and Practice Using C++. I am on the first exercise in Chapter 10, where it says to write a program that produces the sum of all the numbers in a file of whitespace-separated integers. I based my code below on what's used for Exercise 2 of Chapter 10.5. I get an error when the ifstream object is created. Here is the code I am trying to run:

#include "../../std_lib_facilities.h"

int main(int argc, const char * argv[]) {
    // insert code here...

    cout << "Plese enter the input file name: " << endl;
    string iname;
    cin >> iname;
    ifstream ist {iname};
    if (!ist) error("Can't open input file ",iname);

    vector<int> numbers;
    int sum;
    int n;
    while(ist>>n) {
        numbers.push_back(n);
    }

    for (int i=0; i<numbers.size(); ++i) {
        sum += numbers[i];
    }
    cout << sum << endl;

    return 0;
}

Any input I enter is getting error. I tried myin, myin.txt or any other name. The error("Can't open input file ",iname); is from the library created by the author.

I know the file does exist in the same directory with main.cpp and created with TextEdit from Mac using the format for plain text.