If you are using Dokan Lite, Customers can’t sell product right after creating an account on your site. To enable this feature, Administrator will have to approve it manually: Users -> The user name -> Enable Adding Products However, you can allow customer to sell product on your site right after creating an account by putting the following code to functions.php of Child Theme:

add_action('wilcity/after/created-account', function ($userID) {
    $oUser = new WP_User($userID);
    if (in_array('seller', $oUser->roles)) {
        update_user_meta($oUser->ID, 'dokan_enable_selling', 'yes');
    }
}, 10);