Can't save data in WordPress via AJAX (plugin)

I’m working in a new plugin and I want the users use a form to send his/her email. This email would be saved in a custom table in the database.

This is the html form:

<form class="form-inline singup subscribe-database" action="#" method="post" data-url="<?php echo verde_product_info('extend_url') ?>">
<input type="hidden" name="language" value="<?php echo verde_top('language'); ?>">
<input type="hidden" name="database" value="<?php echo $wpdb->prefix; ?>">
<input type="email" name="email" placeholder="Email" required>
<button type="submit" class="btn">Send</button>
</form>

When it is sending, the js code is the next:

var urlplugin = jQuery('.subscribe-database').attr('data-url');
    
    jQuery('.subscribe form').submit(function() {
        var postdata = jQuery('.subscribe form').serialize();
        jQuery.ajax({
            type: 'POST',
            url: urlplugin + '/themes/php/sendmail.php',
            data: postdata, 
            dataType: 'json',
            success: function(json) {
                console.log(json);
            },
            error: function (xhr, ajaxOptions, thrownError) {
                console.log(xhr.status);
                console.log(thrownError);
            }
        });
        return false;
    });

And the file sendmail.php when the error happens is the next one:

global $wpdb;


if(!isset($wpdb))
{
    require_once('../../../../../wp-includes/wp-db.php');
}

switch ($_POST['language']) {
    case 'english':
        $success1 = 'Thanks for your subscription!';
        $error1 = 'Insert a valid email address';
        break;
    case 'spanish':
        $success1 = '¡Gracias por tu suscripción!';
        $error1 = 'Escribe un email válido';
        break;
    case 'french':
        $success1 = 'Merci pour votre abonnement!';
        $error1 = 'Insérez une adresse e-mail valide';
        break;
    case 'german':
        $success1 = 'Vielen Dank für Ihr Abonnement!';
        $error1 = 'Legen Sie eine gültige E-Mail-Adresse';
        break;
    case 'italian':
        $success1 = 'Grazie per il vostro abbonamento!';
        $error1 = 'Inserire un indirizzo email valido';
        break;
    case 'portuguese':
        $success1 = 'Obrigado pela sua inscrição!';
        $error1 = 'Insira um email válido';
        break;
}

function isEmail($email) {
    return(preg_match("/^[-_.[:alnum:]]+@((([[:alnum:]]|[[:alnum:]][[:alnum:]-]*[[:alnum:]]).)+(ad|ae|aero|af|ag|ai|al|am|an|ao|aq|ar|arpa|as|at|au|aw|az|ba|bb|bd|be|bf|bg|bh|bi|biz|bj|bm|bn|bo|br|bs|bt|bv|bw|by|bz|ca|cc|cd|cf|cg|ch|ci|ck|cl|cm|cn|co|com|coop|cr|cs|cu|cv|cx|cy|cz|de|dj|dk|dm|do|dz|ec|edu|ee|eg|eh|er|es|et|eu|fi|fj|fk|fm|fo|fr|ga|gb|gd|ge|gf|gh|gi|gl|gm|gn|gov|gp|gq|gr|gs|gt|gu|gw|gy|hk|hm|hn|hr|ht|hu|id|ie|il|in|info|int|io|iq|ir|is|it|jm|jo|jp|ke|kg|kh|ki|km|kn|kp|kr|kw|ky|kz|la|lb|lc|li|lk|lr|ls|lt|lu|lv|ly|ma|mc|md|mg|mh|mil|mk|ml|mm|mn|mo|mp|mq|mr|ms|mt|mu|museum|mv|mw|mx|my|mz|na|name|nc|ne|net|nf|ng|ni|nl|no|np|nr|nt|nu|nz|om|org|pa|pe|pf|pg|ph|pk|pl|pm|pn|pr|pro|ps|pt|pw|py|qa|re|ro|ru|rw|sa|sb|sc|sd|se|sg|sh|si|sj|sk|sl|sm|sn|so|sr|st|su|sv|sy|sz|tc|td|tf|tg|th|tj|tk|tm|tn|to|tp|tr|tt|tv|tw|tz|ua|ug|uk|um|us|uy|uz|va|vc|ve|vg|vi|vn|vu|wf|ws|ye|yt|yu|za|zm|zw)$|(([0-9][0-9]?|[0-1][0-9][0-9]|[2][0-4][0-9]|[2][5][0-5]).){3}([0-9][0-9]?|[0-1][0-9][0-9]|[2][0-4][0-9]|[2][5][0-5]))$/i", $email));
}

if($_POST['email']) {
    $table = $_POST['database'] . 'verde_emails';
    $subscriber_email = ($_POST['email']);

    if(!isEmail($subscriber_email)) {
        $array = array();
        $array['valid'] = 0;
        $array['message'] = $error1;
        echo json_encode($array); 
    }
    else {
        $array = array();
        $array['valid'] = 1;
        $array['message'] = $success1;
        echo json_encode($array);
        $wpdb->insert( 
            $table, 
            array( 
                'option_email' => $subscriber_email, 
            )
        )
    }
}

When I use the form I get the next error in the browser console:

200
scripts-new.js:43 SyntaxError: Unexpected token < in JSON at position 0
    at JSON.parse (<anonymous>)
    at parseJSON (jquery-1.9.1.min.js:2:4322)
    at Fn (jquery-1.9.1.min.js:4:15175)
    at k (jquery-1.9.1.min.js:4:13895)
    at XMLHttpRequest.r (jquery-1.9.1.min.js:4:18226)

But it works fine if I comment the next code in the file sendmail.php:

$wpdb->insert( 
$table, 
    array( 
        'option_email' => $subscriber_email, 
    )
)

but obviously, the email can’t be saved in the database.

I’m lost with this error and I will be very grateful if someone can help me.

Sorry for my English and thank you very much.