I am in the same situation (beginner to CPP, working through this introductory textbook). I was able to run code as per the above, but failed on a subsequent step which involves string concatenation:

#include "PPP.h"

int main()
{
    cout << "Hello\n";
    string first;
    string second;
    cin >> first >> second;
    //string name = "";
    string name = first + " " + second;
    cout << "Hello, " << name << "\n";
}

Whilst I couldn't get rid of the error message "incomplete type "PPP::Checked_string" is not allowed", I was at least able to get this code to build and run by modifying the definition of "Checked String" in PPP_support.h:

PPP_EXPORT class Checked_string : public std::string {
public:
    using std::string::string; // Inherit constructors

    // Default constructor
    Checked_string() : std::string() {}

    // Constructor to accept std::string
    Checked_string(const std::string& s) : std::string(s) {}

    // Implicit conversion to std::string
    operator std::string() const {
        return std::string(*this);
    }

    // Overloaded subscript operator with range checking
    char& operator[](size_t i) {
        std::cerr << "PPP::string::[]\n";
        return this->std::string::at(i);
    }

    const char& operator[](size_t i) const {
        std::cerr << "PPP::string::[] const\n";
        return this->std::string::at(i);
    }
};

I don't really understand why this works, but it did at least allow me to progress in Chapter 2 of this introductory textbook for beginners.