Hi,
The error message indicates that there is a line of code with "isAdmin()" in the file media/com_hikashop/mail/order_status_notification.created.preload.php
This file is a "per status override" of the "preload" of the "order status notification" email that you have created for the "created" order status. This is not a default file of HikaShop. It is an email override file. So if you've eliminated it, as you said you did, you cannot possibly get this error message.
You must have created it in the past based on a really old version of the preload of this email.
Since then, we've changed the code so that no "isAdmin" is in the preload of that email to be compatible with newer versions of Joomla.
However, since you didn't update your view override, it still is based on the old code which isn't compatible with Joomla 5, hence the error.
The simplest is to rename the file media/com_hikashop/mail/order_status_notification.created.preload.php to something else via FTP.
This way, you'll use again the default preload of HikaShop, which doesn't have the isAdmin code.
However, you'll loose you're customization of the preload. So, you'll likely have to redo it via the interface in System>Emails.