How can I create an array inside a function then use this array to create another array?

Translating code line by line is almost guaranteed to fail. You better do it in two steps: 1) fully understand what code in language A does. 1a) forget about the code in language A. 2) Write the same in language B.

I am not very proficient with python so I start from this:

Basically I want to create a function that returns an array with the y value and an array counting until x.

You declared the function to return a single int. A single int is not two arrays.

Next, this

int l[x];

Is not standard c++ because x is not a compile time constant. Some compilers offer it as an extension, but there is no reason to use it because c++ has std::vector.

Then, this

int board[y, l[x]];

is problematic in multiple ways. l[x] is accessing an element in the array l that is out of bounds. Valid indexes are 0 till x-1 because l has x elements. Accessing the array out of bounds is undefined behaviour. We could stop at this point, because in the presece of undefined behaviour anything can happen. However, y, l[x] invokes the comma operator. It evaluates both sides and results in the right operand. Then you again have the same problem, l[x] is no compile time constant.

In this place I had code in c++, but it turned out that I completely misunderstood what your code is supposed to do. I'll leave the answer and refer you to others for the solution.

There are several problems with your code. The main one is that the Python array area contains objects of two different types: The first is the integer y, the second is the array length. All elements of a C++ array must have the same type.

Depending on what you want to use it for, you can replace the board array with a std::pair. This is an object containing two elements of different types.

Also, in C++ arrays with non-constant lengths must be dynamically created. Either using the new operator or (better) using std::unique_ptr. (Or you might want to use std::vector instead.)

Here's a small C++ program that does something like what you want to do:

#include <utility>
#include <memory>

auto createBoard(int x, int y) {
    return std::make_pair(y, std::make_unique<int[]>(x));
}

int main() {
    auto board = createBoard(5,6);

    return 0;
}

(This will only work if your compiler supports C++14 or newer.)

But this is actually rather much above "newbie" level, and I doubt that you will find it very useful.

It would be better to start with a specification of what your program should do, rather than try to translate code from Python.

EDIT

Same code with std::vector instead of a dynamic array:

#include <utility>
#include <vector>

auto createBoard(int x, int y) {
    return std::make_pair(y, std::vector<int>(x));
}

int main() {
    auto board = createBoard(5,6);

    return 0;
}