If you've tried placing a parking stall some where on the lot to place the car on and it (the parking stall) isn't able to be placed. You may need to go into Build mode > Terrain Tools and flatten out a spot for the parking stall. - Maybe try the Level Terrain option to level out a spot.
After you've done that, try placing the car on the parking stall.
Adding some photos, just in case they're needed. Click on images to see larger view.
Build Mode > Level Terrain Tool - Photo
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Level Terrain - Photo
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