Why const double && doesn't work for lvalue reference?

Explain me, please, how it works? Why double && works for lvalue and rvalue? And why const double && don't work for lvalue?

template <typename U>
void function(U& var) {
    std::cout << var << std::endl;
}

int main()
{
    int var1 = 45;
    function(var1);
    function(45); //error
}

////////////////////////////////////////////////

template <typename U>
void function(const U& var) {
    std::cout << var << std::endl;
}

int main()
{
    int var1 = 45;
    function(var1);
    function(45);
}

////////////////////////////////////////////////

template <typename U>
void function(U&& var) {
    std::cout << var << std::endl;
}

int main()
{
    int var1 = 45;
    function(var1);
    function(45);
}

////////////////////////////////////////////////

template <typename U>
void function(const U&& var) {
    std::cout << var << std::endl;
}

int main()
{
    int var1 = 45;
    function(var1);  // error
    function(45);
}