I just run the sample code from multiprocessing's documentation page:

#https://docs.python.org/3/library/multiprocessing.html

from multiprocessing import Pool

def f(x):
    return x*x


with Pool(5) as p:
    print(p.map(f, [1, 2, 3]))

## end of code

And it cannot run, with the following error messages:
Process SpawnPoolWorker-2:
Traceback (most recent call last):
  File "/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.8/lib/python3.8/multiprocessing/process.py", line 315, in _bootstrap
    self.run()
  File "/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.8/lib/python3.8/multiprocessing/process.py", line 108, in run
    self._target(*self._args, **self._kwargs)
  File "/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.8/lib/python3.8/multiprocessing/pool.py", line 114, in worker
    task = get()
  File "/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.8/lib/python3.8/multiprocessing/queues.py", line 358, in get
    return _ForkingPickler.loads(res)
AttributeError: Can't get attribute 'f' on <module '__main__' (built-in)>

There is already a solution: https://stackoverflow.com/questions/41385708/multiprocessing-example-giving-attributeerror

When putting the function into a separate file and import it in the main file, there is no error at all.

##File: defs.py

def f(x):
    return x*x


##File: run.py

from multiprocessing import Pool
import defs

if __name__ == '__main__':
    with Pool(5) as p:
        print(p.map(defs.f, [1, 2, 3]))