If you want to see notices/messages in the admin area, try ‘Admin Notices’ . See https://codex.wordpress.org/Plugin_API/Action_Reference/admin_notices .
A good tutorial is here: https://www.wpbeginner.com/wp-tutorials/how-to-add-admin-notices-in-wordpress/.
I recommend setting the ‘is_dismissable’ class in the HTML of the message you want to display. That will allow you to close/exit the notice.