Diagnosis
You tried to post negative activities in the activity backflush.
You entered a reference quantity of above or below 0.

System Response
The system only allows negative activities in delta activity backflush.
Delta postings are characterized by the fact that the reference
quantity is zero.
It only makes sense to use negative activities for correction purposes.
You cannot post a negative quantity for total activities.

Procedure
Post a positive activity or enter a reference quantity of zero in the
initial screen.