From the Rent Collection tab, when you are initially setting up Rent Collection terms, you can select ‘Rent Collection Terms’ from the left menu. From there, below the ‘Monthly Rent Amount’ and ‘Rent is Due On’ options, you’ll be able to toggle the ‘Charge a Late Fee’ option. Choose how many days after the rent is due to apply the late fee and the late fee amount.

See here, for information on how late fees are calculated.