When you get this error it usually means that a car has been set in the server_cfg.ini which isn't added to the entry list or vice versa.
For example you set: CARS=bmw_m3_e30;ruf_yellowbird in the server_cfg.ini
Then the entry_list.cfg needs to have the exact same cars, for example:
[CAR_0]
MODEL=bmw_m3_e30
SKIN=Brilliant_red
SPECTATOR_MODE=0
DRIVERNAME=
TEAM=
GUID=
BALLAST=0
[CAR_1]
MODEL=ruf_yellowbird
SKIN=1-Yellowbird-black
SPECTATOR_MODE=0
DRIVERNAME=
TEAM=
GUID=
BALLAST=0
Another problem could be that the car is custom content and doesn't exist in the standard Assetto Corsa installation, in that case see the knowledge base article on how to upload custom cars.
