After the unit Dry Undercoat, the next slot dry undercoat is not avalible. Does make any sens and the production proces stops.
The tooltips seem to be misleading, in that they appear to ask you to place another dry undercoat after another dry undercoat, but that’s not the process. The next slot after dry undercoat is paint finish.
Also, that looks like a highly convoluted paint drying system you’ve got there.
I have noticed that you will sometimes get unexpected missing slots when you have conveyor belts that are not connected anywhere. As soon as I fixed those connections the errors go away. I noticed a few conveyors going to nothing in your screenshot. Try fixing those and it might correct the issue.