Hello Ankit,
the postal code field is not mandatory itself, the message is created by the address consistency check of the central address services used by the business partner. This is why the field does not have the asterisk in the UI.
If the postal code is required or not is defined for each country in table T005. There are 2 flags one for street address and one for post box in the table. This should be the reason why you are getting the error only if an entry in the street fields is made.
If you are in a standalone scenario where eRec is in its own instance you could simply remove the flags for all countries. But I would not recommend this in any other scenario.
Kind regards
Roman