Please try with the updated code
// here is my full code
global $wpdb;
$user = wp_get_current_user();
$mega = $user->user_login;
$age = 19;
// if records were not found in the database
// then insert new row with the details
if ( $age == 19) {
$fillup = $wpdb->insert("markrecord_table", array(
"username" => $mega,
"post_id" => 340,
"counted" => 1,
));
}