Why am I getting a "No viable conversion from 'vector<Country>' to 'int'?

int main()
{
    vector<Country> readCountryInfo(const string& filename);
        
        // Creating empty vector
        vector<Country> myVector;
        
        // Opening file
        ifstream in;
        in.open("worldpop.txt");
    
        if (in.fail()) {
            throw invalid_argument("invalid file name");
        }
        
        while (in) {
            
            char buffer; // Character buffer
            int num; // Integer to hold population
            string countryName; // Add character buffer to create name
            
            while (in.get(buffer)) {
                
                // Check if buffer is a digit
                if (isdigit(buffer)) {
                    in.unget();
                    in >> num;
                }
                
                // Check if buffer is an alphabetical character
                else if (isalpha(buffer) || (buffer == ' ' && isalpha(in.peek()))) {
                    countryName += buffer;
                }
                
                // Checking for punctuation to print
                else if (ispunct(buffer)) {
                    countryName += buffer;
                }
                
                // Check for new line or end of file
                else if (buffer == '\n' || in.eof()) {
                    // Break so it doesn't grab next char from inFile when running loop
                    break;
                }
                
            }
            
            Country newCountry = {countryName, num};
            myVector.push_back(newCountry);
            
        }
        
    return myVector;
    
    }