Diagnosis
You have not assigned a cost center to the rental unit.
In Customizing the cost center is defined as an optional field.

System Response
If you save the rental unit without assigning a cost center, thevacancy costs are posted directly to the rental unit.

Procedure
To assign a cost center, cancel the save function by choosing "Cancel".Then assign the cost center with "Edit -> Cost center..." and saveagain.
If you do not want to assign a cost center, confirm the warning bypressing ENTER.