How to add an error notice conditionally

Question

I am trying to add an error notice to an admin screen. Like this. So. I have a the function which outputs the error:

public function my_error_notice()
    {
        $text = __( 'There has been an error. Error!', 'kea' );
        $output =  <<<EOT
        <div class="error notice">
            <p>$text</p>
        </div>
EOT;
    }

And, in the constructor for my plugin I have:

 add_action( 'admin_notices', array($this, 'my_error_notice' ));

This works and outputs the error message. But - I only want this error message to appear when a specific condition occurs. See below:

I am hooking into user_register and in my callback for that I am doing some extra database work. I have a logged in author. That author has been given the capability to add/register new users. They do this using the normal WP admin screen for an admin to add a new user. In my plugin I have;

add_action('user_register', array($this, 'link_user_to_tutor'));

and

 public function link_user_to_tutor($new_user_id)
    {

        global $current_user;
        $current_user_roles = $current_user->roles;
        $current_user_id = get_current_user_id();
  
        if (($current_user_id !== 0) && in_array('author', $current_user_roles))
        {
            error_log("non admin user adding" . $new_user_id);
            //int or false
            $result = $this->wpdb->insert($this->kea_table_name4, array(
                'tutor_idx' => $current_user_id,
                'student_id' => $new_user_id
            ), array( '%d', '%d')); 
        }
     
        if ($result === false)
        {
            $err = "link_user_to_tutor-" . $new_user_id . "-" . $current_user_id . " wp user was created but not linked to tutor";
            
            $this->mail_error($err);
            error_log($err);
            //I want to print a message to the author/admin here to say that adding the new user has not worked.

        }

    } 

Thanks