How can I solve the error -- error: invalid types ‘int[int]’ for array subscript?

Your function parameter ar is a int*. But when you wrote sum+=ar[i][j] you're subscripting it as if we had a 2D array. You can only subscript it for one dimension like arr[i].

Additionally, row and col are not constant expressions. And in Standard C++ the size of an array must be a compile time constant(constant expression). So,

int ar[row][col]; //this statement is not standard c++

The above statement is not standard c++.

A better way(to avoid these problems) would be to use a 2D std::vector instead of a 2D array as shown below.

#include <iostream>
#include <iomanip>
#include <vector>

//this function takes a 2D vector by reference and returns a double value
double avg(const std::vector<std::vector<int>> &arr)
{ 
    int sum=0;
    for(const std::vector<int> &tempRow: arr)
    {
        for(const int &tempCol: tempRow){
        
        sum+=tempCol;
        //std::cout<<sum;
            
        }
        
    }
    
    return (static_cast<double>(sum)/(arr.at(0).size() * arr.size()));
  

}

int main()
{
    int row, col;
    std::cout<<"How many rows does the 2D array have: ";
    std::cin>>row;
    std::cout<<"How many columns does the 2D array have: ";
    std::cin>>col;
    
    //create a 2D vector instead of array
    std::vector<std::vector<int>> ar(row, std::vector<int>(col));
    
    std::cout<<"Enter the 2D array elements below : \n";
    for(auto &tempRow: ar){
        
        for(auto &tempCol: tempRow){
        std::cin>>tempCol;
        }
    }
    std::cout<<"\n Array is: \n";
    for(auto &tempRow: ar)
    {
        for(auto &tempCol: tempRow)
        
            std::cout<<std::setw(6)<<tempCol;
        
        std::cout<<std::endl;
        
    }
  
    std::cout<<"\nAverage of all the elements of the given D array is: \n";
    std::cout<<avg(ar);
    
    return 0;
}

The output of the above program can be seen here.

The reason you are running into issues is because you are trying to dynamically allocate a 2D array (as opposed to statically).

Static allocation means before the code is even run, the values of [row] and [column] are specified. Example: int ar[35][35] This would create a 35x35 2D array at compile time.

Dynamic allocation means changing the values for [row] and [column] during runtime (after compiling, while running). This will require using the new statement in C++ to create a new array of a specified size at the beginning of your function avg. There is a lot of information about dynamic allocation of arrays in C++, such as this post here