Unfortunately, I've had this regularly from many large and small airports. Probably ~1/month the last 12-18 months. Suggest sending a complaint in through the Avis.com CS link. Send in when the car was returned and include all of the following (if you have them): fuel receipt from nearby, pic of the mileage/fuel gage/time at time of return, pic of the car in the garage, Google timeline showing when you were in their lot, snapshot of the app error showing it wouldn't let you return the car while you're in their lot, boarding pass or receipt for outbound flight proving when you left town (unfortunately, I'm familiar enough with their crap that I now get pictures/screenshots of all of the above when returning a car, whether or not they check it in on the spot). Whenever I do this, they usually return the car within 12 hours with the correct time. Other times, it takes an extra email or two, but they get it returned.