Sixt review: Car not available for pickup

I arrived for a reservation at the ATL rental counter during business hours on 6/6/2023 and was told they didn't have a child car seat and couldn't get me into a car that evening. I was told to wait until 6am or advised to go to another counter to rent a different car. Seeing as how this booking was made 2+ months ahead of time, the car and seat should have been available. When I pressed the counter agent about the lack of availability, he stated that he thought they were just locking up (after 20-minutes of him stalling to find my reservation) and he didn't know if the cabinet could be unlocked where the child seats were kept. He then rudely told me there was nothing he could do, even though the person wh locked the cabinet and had the keys in his hands was now (+10-minutes later) standing next to him.

I can back the next morning to speak with the manager, who said he'd look into it and call me back. I never got a response, lost a day of my rental reservation that I paid for and had to rent another car from a different company so that my family (with two young children) didn't have to sleep in the lobby of the rental counter.