I am trying to find the height of a Binary tree with this code, but it keeps returning 0, can someone tell me why?

You're mixing up two ways of doing things. Either pass the result parameter, using a pointer or an address (int* and &maxheight). In this case there is no return value. But tbh, this is a coding style I've mainly only seen in hardware programming.

Otherwise you can return a result. You already have a return parameter in place, just fix the actual usage of the value because your code ignores it.

Here's the two ways to do it:

Using return parameter:

int heightHelper(Node* root, int height)
{

    if (root->right == nullptr && root->left == nullptr) { //checks if the node has a child
        return height;
    }
    else { //if it has then increase height by 1
        height += 1;
        int maxheight1;
        int maxheight2;

        if (root->left != nullptr) {
            maxheight1 = heightHelper(root->left, height);
        }
        if (root->right != nullptr) {
            maxheight2 = heightHelper(root->right, height);
        }

        // return maximum of the two
        if (maxheight1 > maxheight2) return maxheight1;
        else return maxheight2;
    }
}

int height(Node* root)
{
    // Write your code here.

    return heightHelper(root, height); //root node base case
}

Using pointer parameters. Note that rootheight is local to each branch so don't pass it as a pointer.

void heightHelper(Node* root, int* maxheight_ptr, int rootheight)
    {

    if (root->right == nullptr && root->left == nullptr) { //checks if the node has a child
        if (*maxheight < rootheight) {
            *maxheight = rootheight;
        }
    }
    else { //if it has then increase height by 1
        rootheight += 1;

        if (root->left != nullptr) {
            heightHelper(root->left, maxheight_ptr, rootheight);
        }
        if (root->right != nullptr) {
            heightHelper(root->right, maxheight_ptr, rootheight);
        }
    }
}

int height(Node* root)
{
    // Write your code here.

    int maxheight = 0;


    heightHelper(root, &maxheight, 0); //pass the addresses of variables

    return maxheight;
}