Hi,
This means that you've customized the "preload" section of some of your emails in the menu System>Emails. And you did this with an old version of HikaShop. And thus the PHP code in your customized "preload" is old code which doesn't support Joomla 4.
The simplest solution is to delete the file where the error happens so that you'll use the default preload of the email. However, you might lose some customization you had in place there. Not knowning what is in your customized preload, I can't say for sure what is the solution.
However, I think it's probable that you have the code
on the line of the error. And in that case, you can just replace it by:
hikashop_isClient('administrator')
and it will fix the error.
You can do that via FTP or editing the order creation notification email preload via the menu System>Emails.