Errors after upgrading to Joomla 4

Hi,

The error "Call to undefined method Joomla\CMS\Application\SiteApplication::isAdmin()" doesn't correspond to what I can see on your screenshot.

What I can see on your screenshot indicates a problem with the Greek language pack you're using on your website. It's apparently an old language pack made for Joomla 3, not Joomla 4. You'll have to update it.

Regarding the "Call to undefined method Joomla\CMS\Application\SiteApplication::isAdmin()" error, I can't provide a direct solution. This is only the abbreviated error message so it doesn't point at where the problem is. It is probably in some kind of customization (an email customization, or a view customization, or an old plugin integrating with HikaShop which needs to be updated). You need to activate the "debug" setting of the Joomla configuration and then reproduce that error in order to get the full error message. Then, we'll be able to tell you what to do.

Hello,

Your issue come from 2 things :
1. Your 2 Worldpay payment plugins weren't php 8.2 AND as you didn't use it, you can unactivated its in your Joomla plugins manager
2. Your current template, with a default template your checkout works as expected.

=> You have to contact the template creator support to solve your last issue.
Regards