Not the suspension strut
No, It’s within the build. My clue was that it’s near the piece that Jabin guessed.
I’m sorry I was just writing random guesses. Here’s another one:
Is it the red rectangular thingy that is under the gray thingy with the blue and beige round thingies inside it?
Are you looking at the original pic?
No, the one zoomed in I look at now.
If you mean the one with the light blue lines that together form a box around the rectangular red object, then yes that is the one I mean 
It’s not it.
Oh. <………>
It seems that dots don’t work they have to be letters.
So… Is it the black long stick that goes straight through the red thing, the gray thing?
No
Is it the white thing in bottom?
Do you mean the gear or the beam? Actually it doesn’t matter because it’s neither
Is it the wheels?
No.
I think I’ll give the answer now.
It was the black and red pieces because they’re supposed to be replaced by the grey bit that was to the side but the program was bugging and wouldn’t let me place it.
@Jabin_Quaken you go because you accidentally circled part of it in one of your guesses.
How diabolical.
@Jabin_Quaken …hey¡ 
you go
The trashbag in the back