I encountered a problem that there is no IMEI on the back cover

Read this write-up on the common issues with used-equipment purchases:

The All Too Common SAD Reality of Buying a used iPad/iPhone - Apple Community

The IMEI is also worth a check, but the “check your IMEI” sites could well just be looking to clone an IMEI... I think this is the legitimate IMEI-checking site:

https://stolenphonechecker.org/spc/consumer

The carriers can also check.

This document explains where you should look for IMEI number of your iPhone. https://support.apple.com/en-ie/HT204073

Models for some regions are not required to display the IMEI on the back. Why do you feel this is a problem? Does the phone work?