If you want to assign base rent surcharges to a specific period, you must first define them.
Procedure in the FM Portal
Path to the starting point: System Control - Property Management - Leasing - Master Data/Statistics - Base Rent Surcharges - Base Rent Surcharges for Rooms
Create a new record in the table.
Fill in all required fields. If necessary, use the form structure (accessible via the help icon in the FM Portal) as a guide.
Click Save in the toolbar.