Personally, I struggled with assembling cars for a long time and did not understand why the achievement was not counted, but my friend told me a great way to find out what the car wants without racking your brains, and I want to share this method with you.

First, open the game and go to the car that does not count or you do not understand what is missing in it. I will show on the example of an already assembled triclops, where we assume that we do not have a gas pedal, but we do not know about it

and an unfinished golf.
Now go to the folder at C:\Users\YOUR_USER\AppData\Local\DriveBeyondHorizons\Saved\Logs and look for the file "DriveBeyondHorizons.log"

But do not open this file yet.
Now tear off any spare part from the car, in my case it will be the door.

and immediately put it back in place.

Now open our log file and scroll to the very bottom, right to the very bottom, and look for a line similar to this one, where the name of our car is written and the problem is described.

Thus, we found out that we do not have a gas pedal in the car. ( and we didn't know!!! ) We find the right gas pedal and put it in place, we get the achievement. (Hurray....)
The same with similar parts. Visually, the Triclops has the same brake and clutch pedals

At first glance - everything is correct, but where is my achievement??? And we'll see what the log tells us.

And the log tells us that we put them in a jumble!( and we didn't know!!! x2) We find the right pedal and put it in place or swap them around, we get the achievement. (Hurray....)

Didn't get the achievement?

Don't be sad, repeat the process described above again and see what else is missing. Keep in mind that the log only writes about one missing part, and until you put the missing part back in place, you won't see what else is missing. Now I'll show you this using the example of a Golf - a long-term construction project.
Repeat the same process with the door torn off

And go to the log and see that we have a missing windshield

But the log doesn't tell us about the missing left rearview mirror or trunk glass (which I don't have either, but its absence is not visible on the screenshot), so be careful!

I hope my guide helped you figure out your car and you were able to get the coveted achievement