Booking App Overcharged Me for a Rental I Never Got – How to Fight Back

We arrived at Stuttgart Airport in Germany using the Booking app. They charged me a payment one day earlier from my card. At the location they asked for an additional €500 fee, which we accepted. Afterwards they said they would also charge an extra $707 as a security deposit. The rental agency on-site, Flex GO, told me there was no car, and that there were a lot of excuses and tricks. I was unable to take the car. Even though I couldn’t get the car, Booking still hasn’t refunded my $350 USD (≈₺14,826) payment. Is this possible? They say “you should have called from there.” I have sent dozens of emails, but there is no one to address my case.