Diagnosis
V1 is already assigned to v2 (in the period from v3 to v4).
Multiple assignment during the same period is not allowed in this case.

Procedure
Assign a different object. If this involves the assignment of a rentalobject to an occupancy contract, and you only want informational assignment, then set theindicator.
Setting the indicator is only possible if this is allowed in Customizing for this assignment.