undefined method Joomla\CMS\Application\AdministratorApplication::isAdmin

Hi,

Please try again with the "debug system" setting of the Joomla configuration activated.
That way it will provide an error message with more information so that we can better understand where the problem is coming from.
For example, it could be that the issue is that another extension is loaded when HikaShop is being updated and that's what is causing the problem. Without the full error message, it's hard to say what it could be actually.

Hello,

As requested previously by Nicolas we will need to have more information around this error for this follow me step by step :
1. Go to your Joomla Global configuration
2. Then in System tab AND in Server tab, reproduce my settings shown in my screenshot :
i.ibb.co/XbPmk8j/joomla-debug.png

3. Then reproduce the situation to show the error which this time will be much more detailed.
=> Make us a screenshot to allow us to see the error with all these details.

4. If you still don't see more details on the error message, it might be your template hiding the details of the error. So rename temporarily the file templates/YOUR_TEMPLATE/error.php and try again with the debug/error reporting activated.

Awaiting your elements to progress on this subject.

Note that this error message is a typical message of a Joomla 4 incompatible extension, but there have been thousands of merchants already using HikaShop on Joomla 4. So it's hard to say what to do as the "undefined method Joomla\CMS\Application\AdministratorApplication::isAdmin" message in itself is too generic to say anything.
One possibility I can see is that you had installed HikaShop and other plugins for HikaShop in the past on Joomla 3. You uninstalled HikaShop and migrated your website to Joomla 4. Now, you're trying HikaShop again, and when you install it, the old plugins that were not called anymore are called by HikaShop, but since they were not updated or uninstalled, they generate that error. But again, we need the full error message to tell you which plugin it would be, supposing this theory is at play here.

Regards