Notice: Undefined index: mtral_field_subscriber

You can simply remove that plugin, and create that redirection youself.

You can use the login_redirect hook, add this into your theme’s functions.php

function redirect_admin( $redirect_to, $request, $user ){

    //is there a user to check?

    if ( isset( $user->roles ) && is_array( $user->roles ) ) {

        //check for admins
        if ( in_array( 'administrator', $user->roles ) ) {

            $redirect_to = get_site_url().'/a-page/'; // Your redirect URL
        }
    }

    return $redirect_to;
}

add_filter( 'login_redirect', 'redirect_admin', 10, 3 );