@priyankabansal Are you able to specifically call just the record in question in your API call and see if it provides you with any errors? That might help narrow down the issue. Since you mentioned you were able to fetch all but one record from the GS User object, that tells me it’s likely not a payload structure issue but I am not an API expert by any means.

System Administrator is the “User” in whose name updates from the Rules Engine or the Connector jobs occur.

I would guess, but don’t know for certain, that the “System Administrator” User is unique among Users, and may not live in and behave like all the traditional Users representing actual people. Hence, I wouldn’t be shocked that it behaves differently with respect to API calls.