When a new Requisition is created, it's values are set as for the resource in the project, this includes the Start Date, Finish Date and Average Rate in percentage.
Changing the Average Rate percentage to 0 saves fine but any save afterwards will show a system error.
STEPS TO REPRODUCE
1. Create a new Requisition for a resource within a Project:
Team - Staff - mark a resource - Actions Menu - Create Requisitions
2. Click on Create, a new Requisition is created.
3. Open the Requisition ( Team - Requisitions), change the Average Rate percentage to 0 and save.
4. This will save fine, the row is deleted and the 0% is saved, and when you open the Requisition again, you'll see that the requested amount is deleted, but any new save will now show an internal system error.
It is legal to have a requisition with no segments, so the system error should not occur.
In the app-ca.log you'll see:
ERROR 2018-07-30 17:32:24,614 [http-nio-14001-exec-41] niku.xql2 (clarity::79598197__14A8B09E-A6EA-4423-B7E4-6E83074D6457:resource.requisitionSave) Internal Processing exception
This was reported as defect DE43020 and is fixed in 15.5.1