How to get psid on Facebook Messenger on iPhone devices?

There is a messenger bot developed for page LeBay. It is an order processing support for messenger users. Customers can select products and place the order on webview and it returns receipt with total amount and product description. This processing is working well on Android devices but on iPhone. iPhone Customers can place order but the system failed to give receipt back to the customer. When I check the log, It says fail to get psid to return message back. (without psid, Bot can't send message back to current user.) How can I solve this issue for iPhone users. Does it cause beacause of ios 14 or Messenger for iPhones?