Chance ACF Field with Javascript over a php function

Solution:

You cannot call a PHP function directly with JavaScript like that. PHP is executed by the server and JavaScript is executed by the browser (in this context anyway)

To achieve what you want here you need to use Ajax. Which is the front-end making a http request to the server and then the server will call your PHP function and return an appropriate response.

Creating the Ajax endpoint for WordPress

<?php
// functions.php

add_action('wp_ajax_call_feld_update', 'feldUpdateCallback');
add_action('wp_ajax_nopriv_call_feld_update', 'feldUpdateCallback'); // nopriv for unauthenticated users

/**
 * Ajax call for updating bewerber_einstufen field
 *
 * @return void
 */
function feldUpdateCallback(): void {
}

So now you have an ajax endpoint you can call from your JavaScript. Now you need to setup the front end and have WordPress create a nonce for you, which is essentially the way you ensure the request came from the front-end of your own website.

<script type="text/javascript">
var wp_nonce = '<?= wp_create_nonce('updating-field-nonce'); ?>';
var ajax_url = '<?= admin_url('admin-ajax.php'); ?>';

// Ninja Table Loaded initially
jQuery(document).on('ninja_table_loaded', function (event, $table, settings) {
    // your code was here
});
</script>

Now the JavaScript variable wp_nonce will have have the nonce stored in it and the ajax_url will have the url for ajax stored in it. This is normally achieved with wp_localize_script however this is a simplified example for you.

Now you can create your ajax request like so

    jQuery(document).on('ninja_table_loaded', function (event, $table, settings) {
        console.log('ninja_table_loaded');
        let changeButton = document.querySelectorAll('select');
        var acfVersion = acf.get('acf_version');
        //alert(acfVersion);    
        for(let i=0; i < changeButton.length; i++){
            changeButton[i].addEventListener("change",function(){
                let rowidparent = this.parentElement.parentElement;
                let rowid = (rowidparent.querySelector("p").innerHTML);
                console.log(rowid);
          
                // ajax request
                jQuery.ajax({
                   type: "POST",
                   url: ajax_url,
                   data: {
                      action: 'call_feld_update', // the action is set when you added wp_ajax_ at the beginning of this answer.
                      postId: rowid,
                      value: 'eingeladen',
                      nonce: wp_nonce
                   },
                   success: function (response) {
                      console.log(response);
                   }, 
                   error: function (jqxhr, textStatus, errorThrown) {
                       console.log(errorThrown);
                   }
                });
            });
        };
    });

With that, WordPress will call the feldUpdateCallback function when the ajax request is sent. Now we update the PHP function that was created before so it can deal with that request…

// functions.php

/**
 * Ajax call for for updating bewerber_einstufen field
 *
 * @return void
 */
function feldUpdateCallback(): void {
    // nonce - The key you set when sending the request from ajax
    // updating-field-nonce - The parameter for the wp_create_nonce function
    if (!wp_verify_nonce($_REQUEST['nonce'], 'updating-field-nonce')) {
        // The string we received does not match the nonce that was set on the front by WP 
        die('nonce validation failed');
    }
    // now you can deal with the request...
    feldUpdate(
        'bewerber_notiz',
        sanitize_text_field($_REQUEST['value']), // https://developer.wordpress.org/reference/functions/sanitize_text_field/
        sanitize_text_field($_REQUEST['postId'])
    );
    die('successful');
}