You have not shared the file so I can't view it and debug it. Upload the file on a file uploader and give the link./// ! This is an error that is likely related to upgrading PHP version above 7.3 !
// Find this line of code in the file
if (isset($this->session->data['shipping_address'])) {
// Replace it with this line of code
if (isset($this->session->data['shipping_address']['country_id']) && isset($this->session->data['shipping_address']['zone_id'])) {
/// I hope that this helps you! Happy coding <3