Unable to convert List<List<int>> to return type IList<IList<int>>

Just change the declaration of your result to List<IList<int>>.

List<T> implements IList<T>, but List<List<T>> does not implement IList<IList<int>>. Generic parameters are not covariant or contravariant unless defined that way and IList<T> is not, so the type must match exactly.

public IList<IList<int>> LevelOrder(TreeNode root)
{
    var result = new List<IList<int>>();
    var que = new Queue<TreeNode>();

    //if(root==null) return result;

    que.Enqueue(root);
    while (que.Count != 0)
    {
        int n = que.Count;
        var subList = new List<int>();
        for (int i = 0; i < n; i++)
        {
            if (que.Peek().left != null)
                que.Enqueue(que.Peek().left);
            if (que.Peek().right != null)
                que.Enqueue(que.Peek().right);
            subList.Add(que.Dequeue().val);
        }
        result.Add(subList);
    }
    return result;
}