You are using the slim version of jQuery, which doesn’t include the Ajax methods (in your case $.post()).
Use the non-slim build, available here: https://jquery.com/download/, such as:
https://code.jquery.com/jquery-3.5.1.min.js
From jQuery 3.0 release post:
Slim build
[…] Sometimes you
don’t need ajax, or you prefer to use one of the many standalone
libraries that focus on ajax requests.[…] Along with the regular version of jQuery that includes the
ajax and effects modules, we’re releasing a “slim” version […], it excludes ajax, effects, and
currently deprecated code.