Don't look at the city report, look at the Building window's Building tab.
What is the plastic factory's blueprint ID number?
Does it have enough commodity storage to store the product from 128 workshops?
Each workshop makes 50 plastic plus the extra from having computers. So that would be between 6400 and 7000 plastic per run. As a workaround try lowering the number of workshops and see if it works.
It is a known issue that buildings with too little commodity storage to store the product from a manufacturing line can't make anything at all. I believe we have a bug report for it somewhere.
EDIT:
This is probably duplicate of (Bug report) Processes don't output if a single run exceeds storage.
But this thread is not really worth merging with that one.
What is the plastic factory's blueprint ID number?
Does it have enough commodity storage to store the product from 128 workshops?
Each workshop makes 50 plastic plus the extra from having computers. So that would be between 6400 and 7000 plastic per run. As a workaround try lowering the number of workshops and see if it works.
It is a known issue that buildings with too little commodity storage to store the product from a manufacturing line can't make anything at all. I believe we have a bug report for it somewhere.
EDIT:
This is probably duplicate of (Bug report) Processes don't output if a single run exceeds storage.
But this thread is not really worth merging with that one.