Do your rectangle lights point in the right direction or why is everything getting black?
You can render your image with alpha channel (environment switched off with "Is Visible") and create a gradient afterwards in photoshop. Like that, you will get a shadow on the shadow plane together with your car and the rest will be transparent. Please take a look in the image attached with the green oval highlight.
Hope this helps!
Florian
Take a look, I shared a garage scene for free use. Maybe you can use it for your render.
Here is how to change the default black to grey background of the dome to other colors (you could change them both to the same color for a single-colored background):
*Hide the Env. Dome
*Edit > Prefs > Render Options > Visualization Advanced > Background
*Click on 1, Change Color (top of Render View)
*Click on 0, Change Color (bottom of Render View)
*Apply
To revert: Set Default
Sorry for the late reply.
Thank you so much for the help.
I will download ur file to test it. ~!