Plugin use of ajax/jquery depending dropdown

1. Does the get-contacts.php script work ok if you load it directly, e.g. in your browser or from the command line, or in some test javascript?
2. Maybe you don’t have the path correct. Instead of providing a partial path you could provide an absolute path. Is the reuse directory in the root of your web space? If so set url like this:

(Edited for the URL you provided – this is probably how you should set it)

url: '/wp-content/plugins/harriecrm/admin/get-contacts.php',

Or for debugging you could also set the entire path, wherever it is, e.g.:

url: 'https://yoursite.com/somewhere/reuse/get-contacts.php',

Let me know if this helps