Nonces

ERROR | Processing form data without nonce verification

<?php
if ( isset( $_POST['test_key'] ) ) {
    $test_key = $_POST['test_key'];
?>

WPCS is telling you that you are processing input data without using a nonce to guard against CSRF.

<?php
if ( isset( $_POST['test_key'], $_POST['test_nonce'] ) && wp_verify_nonce( sanitize_key( $_POST['test_nonce'] ), 'test_action' ) ) {
    $test_key = sanitize_text_field( wp_unslash( $_POST['test_key'] ) ); 
    // ...
?>

https://github.com/WordPress-Coding-Standards/WordPress-Coding-Standards/wiki/Fixing-errors-for-input-data#nonces