The problem comes from various things that are not compatible with Joomla 4 on your site when I debug:
1/ Your override for AcyMailing module in template, by default, this template doesn't override this module so you should check this override code again to update it compatible with J4: \templates\gk_magazine\html\mod_acymailing_ (I just renamed this folder)
2/ Hikashop on your site is not ready for J4: you should update it as well.
I also see you have overrides for K2 and other 3rd party extensions there, K2 is not ready for J4 so it will cause error.
You should check your whole site again and make sure that you have all other extensions ready for Joomla 4 first as GK Magazine is compatible with J4 already.
P/s: you should be noticed all of this incompatible stuffs in pre-check update steps when you update site to J4
