You need housekeeping staff in order to keep all the buildings on the Island up and running. So when you're planning a new building, it pays off to make sure you have enough personnel.
To increase the number of staff, it is necessary to improve the Administrative Building. You can also use special accessories that reduce the required number of staff.
If the Administrative Building is already at the maximum level, but you do not have enough staff for a new building, then you need to move one of the buildings to the warehouse and replace it with a new one with better characteristics, since the success of the Island's development lies in the constant updating and management of available resources and buildings.