TypeError: Cannot read property 'classList' of null

In my case I forgot to add a "." on my querySelector to select a class. If you are trying to select a class or an ID, check that you have the appropriate punctuation. For example:

const someclass = document.querySelector('.someclass');

To select an ID:

const someid = document.querySelector('#someid');

Also make sure that you spell the attribute that you trying to select correctly. Let's say for example you would like to select input but you misspell it on the selector below, it wont work.

Wrong one:

const input = document.querySelector('inputo'); 'inputo doesn't exist.

Correct one:

const input = document.querySelector('input');

It means that document.getElementById("lastName") is returning undefined and you're trying to call classList on undefined itself.

In your HTML input has the attribute name which equals lastName but there is no actual id="lastname"

Either add the attribute id to your input or change getElementById to getElementsByName.

Note that getElementsByName doesn't return a single item.