Cannot access a file in the theme (twentynineteen-child) folder

Could you post your code… its better way to help you.

So, I guess you have html form ?

An idea :

Don’t specify any path on your form and let manage it into your functions.php

add_action('init', 'my_form_process');
function my_form_process(){
  // Do nonce check, Post check...
}

Have look for Nonce security check : Nonce WordPress page