On all the levels you'll find colored paint lines somewhere on the floor, they are usually in a square shape with some text along the borders of it, it looks kind of like a loading zone marking.
It can come in blue, orange, yellow and green, and usually has things written on it somewhere, like "Barrels", "Crates", "Nitrogen".
The easiest one to find and look at is in Splatter Station, they are right there where you start.
When stacking stuff in the area which wants those things(Crates in a crate area), make sure they are upright, you can align things upright by using the left mouse while holding it to be sure.