I receive this error "Class "JRequest" not found" after upgrade from Joomla 3.10.11 to 4.2.6
This error is displayed when some menu items are selected from the frontend.
It due to a 3rd party extension on your site called: Content - Ignite Gallery plugin. It looks like it's not compatible with Joomla 4 yet.
I temporarily disabled it and your site is working now.