If Symantec Endpoint Protection (SEP) Mobile does not have Mobile Device Management (MDM) integration, it does not display the Serial number and Hardware serial for the device under Device details on SEP mobile console.
There are some strict restrictions on Android 9.0 and above has restricted access for both IMEI and serial no. It is available only for platforms and apps with special carrier permission.
SEP Mobile app reporting to the SEP Mobile console.
This is working as designed.
The Serial number and Hardware serial number will be displayed in SEP mobile Console only if it is integrated with MDM.