We request user to send the screenshot with error message: Insufficient Privileges - You do not have the level of access necessary to perform the operation you requested. Please contact the owner of the record or your administrator if access is necessary.
From a quick discussion with user, here are few items we notice:
- Error is not happen to all lookup record, some of the lookup record do not throw error
- User profile has Create permission in both objects, and user able to create new record
- Relationship between this two objects is Master-Detail, and Sharing Setting Organization-Wide Defaults for master object is Read-Only
A quick thought, is there any Roll-Up summary field in parent to that child? If yes, this lookup will do 'edit' activity to parent -- negative, no Roll-Up summary field in parent to that child.
Finally: we found the issue: this is because Master-Detail field in the child object to parent is set to Sharing Setting = Read/Write: Allows users with at least Read/Write access to the Master record to create, edit, or delete related Detail records.
Once, we change this to Read Only, everything is working well.
Reference: Master-Detail relationship (roll-up summary)