Plastic is unlocked with Plastics 1 and that only requires Automation Tech packs to research however to unstack it by hand or with a beltbox you will need Stacking 2. This could be fixed by moving the recipe from Stacking 2 to Plastic 1. The same problem is present with copper wire, iron gear wheel, iron stick and simple circuit board, also located at Stacking 2 instead of Stacking 1 or the other appropriate technology.

Hello Scipio9001,
im very sorry to say this but that is something i cant do about it,
since plastic bar, iron gear wheel, iron stick and simple circuit board are vanilla items
i can advise you to go ask here: https://mods.factorio.com/mod/deadlock-beltboxes-loaders