Hello @Ellen99,
You have a cleaning fee set so it is charged by Airbnb for all reservations.
You can explain to the guest that it's for the time and expense that you go to in cleaning the place before they arrive. It's not meant to cover cleaning during the guest's stay.
They expect to find the place clean and your cleaning fee ensures that you're paid for doing that.
The guest doesn't have the choice to forego the cleaning charge. When they arrive they've already cost you the materials and time for cleaning.
Steve.