Hello,
Session Expires issue caused by WooCommerce Ultimate Gift Card plugin https://prnt.sc/26my2ng Once plugin is disabled then everything works ok.
Contact plugin author and ask to fix that error. I suppose that error in our files could be caused because of that plugin too because the checkout process is failed. So, fix the mentioned error of the mentioned plugin and then let me know if you face any error related to our theme or core plugin after that.
Regards