Why won't these lambda functions be accepted?

Can someone explain why the lambda functions in the commented-out lines will not compile?

#include <iostream>

struct Object {
    int num;

    void foo();

    template <typename... Args> 
    void fooHelper (int(*f)(Args...), Args... args) {std::cout << (*f)(args...);}

};

int sum (int x, int y) {return x + y;}

void Object::foo() {
    fooHelper (sum, 3, 4);
//  fooHelper ([](int i, int j)->int {return i + j;}, 3, 4);
//  fooHelper ([this](int i, int j)->int {return i + j + num;}, 3, 4);
//  Object o{8};  fooHelper ([this, o](int i, int j)->int {return i + j + num + o.num;}, 3, 4);
}

int main() {
    Object o{5};
    o.foo();  // 7
}

But fooHelper (sum, 3, 4); does compile. How to make the three lambda functions work inside Object::foo()?