Why does jQuery mask say it's not a function?

Jquery mask is a plugin. You can directly add this line in your HTML if you want to use a CDN version:

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery.mask/1.14.10/jquery.mask.js"></script>

Another way is to use a package manager like npm or bower. To accomplish that, follow the instructions in the README.md file of the project.

Change this line from

$(document).ready(function(){

to

$(document).ready(function($){

jQuery itself does not provide functionality for masking an input. You can use one of the plugins avaiable for it.