First, make sure the "Restrictions" box is checked in the Rates module:
Then you can click on "minimum number of nights" on the left under the room type name:
set minimum number of nights:
We do not recommend using the surcharge for accommodation shorter than the minimum number of nights, it is not supported by any world accommodation catalogs.
A more suitable solution than a negative motivation with a surcharge for short accommodation is a positive motivation in the form of a discount for a longer stay