Few of the most common problems that occurs in Update set:

1.?? Error type: Missing object

Example: “Could not find a record in sys_ui_policy referenced in this update”

2.?? Error type: Collision

Example: ”Found a local update that is newer than this one”

3.?? Error type: Uncommitted update

Example: “Could not find a table field (u_case.u_reference) referenced in this update, but did find it in another uncommitted update set

4.?? Error type: Table to be deleted has data

Example: “Found a row in the table that is going to be deleted”

5.?? Error type: Application scope validation issue

Example: “Could not find a record in sys_scope for column sys_scope referenced in this update.

for more details you have to follow the below link

https://www.cernasolutions.com/servicenow-update-set-errors-and-how-to-get-around-them/