Button[] btnArray = new Button[drinks.Length];
You have only created an array holding Buttons, but you haven't create individual Button
instances.
You have to create the actual Button before adding them to the Controls and change their properties, something like:
for (int x = 0; x < drinks.Length; x++)//add to form
{
btnArray[x] = new Button { Text = drinks[x].productName };
this.Controls.Add(btnArray[x]);
}