When you call
document.getElementsByClassName() that is returning an array of elements, but when you use the result, you're treating it like a single element (
classList is defined on an element, not on an array of elements). You'll need to go through the array and check each element in it until you find the one you're looking for.
Once you've found the right div, you'll need to find a way to get the input from inside it. Be careful, though, if you use something like
getElementsByTagName("input") even though there's only one input inside each div, you'll still get an array (with one element in it).
I hope that helps!