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This warning relative to the Booking Calendar MultiUser version.
You can see this warning, because your WordPress user was not activated as regular user in Booking Calendar (have not own booking admin panel) at Booking > Settings > Users page, and this user is not the super booking admin user.

By default the first user in your website, its Super booking admin user.

1) Please login to admin panel with this "first user".
It can be that currently you have logged in with administrator user, that have ID as 5 (or any other). But your WordPress website, also have other administrator users with ID, like 1 or 2. If true, so then you need to login with User with login ID = 1

After this you will see access to the booking admin panel. Then you can activate the booking admin panel for other WordPress users at the Booking > Settings > Users page.

2) In case, if your current user do not have access to the Booking > Settings > Users page, and you do not know login details of your first user, so you can make small customization for activating your actual user as super booking admin user.

You can make this fix.

Firstly please recheck ID of your user. You can open the WordPress > Users page, and then near specific user you can click on edit button, then in browser URL you will see parameter user_id
which will show Id of your user, let say that this ID of your current user is 3

So then, please make this fix.

Please open this file ../{Booking Calendar Folder}/inc/_mu/multiuser.php

( you can check how to edit files in WordPress menu in this article https://wpbookingcalendar.com/faq/how-edit-file-in-wp-menu/ )

then find this code:

$this->super_admin_id = array( $admin_id );                             // ID of Super Administrators

and replace it to this code:
$this->super_admin_id = array( $admin_id , 3 );           // ID of Super Administrators