Sixt Car Rental Cancellation Fee Charged Despite No Available Vehicle at Stuttgart Hbf

I went to Sixt at Stuttgart Hbf to rent a car, but there were no vehicles available. The representative there told me that a car was available at another branch and made a reservation for me. However, when I arrived at the other location, there were no suitable cars available, and the only options were at a higher price. I decided to cancel the reservation, clearly stating that I no longer wanted the car. Despite this, I received an email today saying that I have to pay a cancellation fee. I don’t understand why I am being charged a penalty for a reservation I canceled due to their inability to provide the vehicle. I would like an explanation and resolution to this unfair charge.