So, work backwards from the point of the error. Why does button_dec not refer to an object? Probably because the call to parent.firstChildElement didn't return an element, but instead couldn't find a child element. Why did that happen? Probably because your call to getElementsByClassName didn't return anything, or didn't return the element you think it did.
You'll have to insert console.log statements after each step in the code to see what it returned, like this:
const parent = document.getElementsByClassName("button-container");
const button_dec = parent.firstChildElement;
const button_reset = button_dec.nextSibling;
const button_inc = button_reset.lastChildElement;