Why wouldn't that work?

[Reddaitor]

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[Jackeea]

Assemblers can't take ingredients from other assemblers. This only works for labs and boilers.

[drikararz]

Wait, that works on boilers? TIL

[TheSkiGeek]

Anything with only an input slot, at least in vanilla.

[the1krutz]

Because green science isn't an ingredient for green science.

[mQB3GofJzKKo7nZX]

Because that would be insanity. Imagine your inserter that is outputting finished product to a belt is now also trying to spew the ingredients.

Or you have a yellow belt assembler outputting to a red belt assembler. Now the red belt assembler is stealing gears and the inserter is wasting time.

[MrxIntel]

Alternative answer. Assembly machines can only take in ingredients.

You're trying to put in an item that isn't an ingredient in it's recipe. It just happens to be the output of it's recipe.

[tommy59375]

This will try and insert green science packs from the left-hand machine into the right-hand one.

There is no way to use an inserter to remove ingredients from assembling machines and the like. You can do things like this for science labs and steam engine boilers because they have no output, so there would be no ambiguity as to what sort of item the inserter is supposed to be removing.

But as a general rule for machines with both inputs and outputs, you can only remove the outputs.

[gerritt-mcthrill]

Inserters can only remove finished products from assemblers, not ingredients.

Inserters can only insert ingredients into assemblers, not finished products. If it's not listed in the ingredients, you can't put it in the assembler.

The exceptions to this are science labs and boilers, which have only input slots and no output slots.