Hello Frederick.

Thank You for Your purchase.

There is probably a conflict with the bootstrap.js files.

Please go to wpDataTables main Settings on Your wp-admin page. You'll see Include full bootstrap front-end and Include full bootstrap back-end. Since none of the drop-downs are working on back-end, please disable both sliders, hit Apply and refresh Your page. That should solve the issue.

If, however, that doesn't work, please try this:

Go to Plugins page and disable wpDataTables.

Then go to Plugins/Editor on Your wp-admin page.

In the top right corner select wpDataTables and go to source/class.wdttools.php.

In that file, around line 900, You'll find this code:

if (!is_admin() && get_option('wdtIncludeBootstrap') == 1) {
         wp_enqueue_script('wdt-bootstrap', WDT_JS_PATH . 'bootstrap/bootstrap.min.js', array('jquery', 'wdt-bootstrap-select'), WDT_CURRENT_VERSION, true);
        }else if (is_admin() && get_option('wdtIncludeBootstrapBackEnd') == 1){
            wp_enqueue_script('wdt-bootstrap', WDT_JS_PATH . 'bootstrap/bootstrap.min.js', array('jquery', 'wdt-bootstrap-select'), WDT_CURRENT_VERSION, true);
        }else{
            wp_enqueue_script('wdt-bootstrap', WDT_JS_PATH . 'bootstrap/noconf.bootstrap.min.js', array('jquery', 'wdt-bootstrap-select'), WDT_CURRENT_VERSION, true);
        }

Go ahead and comment the last two wp_enqueue_script lines, so the code looks like this:

if (!is_admin() && get_option('wdtIncludeBootstrap') == 1) {
         wp_enqueue_script('wdt-bootstrap', WDT_JS_PATH . 'bootstrap/bootstrap.min.js', array('jquery', 'wdt-bootstrap-select'), WDT_CURRENT_VERSION, true);
        }else if (is_admin() && get_option('wdtIncludeBootstrapBackEnd') == 1){
          //  wp_enqueue_script('wdt-bootstrap', WDT_JS_PATH . 'bootstrap/bootstrap.min.js', array('jquery', 'wdt-bootstrap-select'), WDT_CURRENT_VERSION, true);
        }else{
          //  wp_enqueue_script('wdt-bootstrap', WDT_JS_PATH . 'bootstrap/noconf.bootstrap.min.js', array('jquery', 'wdt-bootstrap-select'), WDT_CURRENT_VERSION, true);
        }

Hit Update File on the bottom of that page; activate wpDataTables from the main Plugins page again, and that should do it.

Please let me know if You managed to resolve the issue.

Best regards.