Multiprocessing code executes properly under PyCharm with Debug button but gets error "Can't pickle <function add ..." with Run button

Question

Environment:

• macOS Big Sur 11.2.3
• Python 3.8.10
• PyCharm 2021.2.1 Professional Edition

When I using multiprocessing module with python3.8.10 and click Debug button run:

import multiprocessing


def add(a: int, b: int):
    return a + b


if __name__ == '__main__':
    pool = multiprocessing.Pool(2)
    multi_result = []
    # begin
    for i in range(20):
        multi_result.append(pool.apply_async(func=add, args=(i, i + 1)))
    pool.close()
    pool.join()
    # print result
    for _r in multi_result:
        print(str(_r.get()))

I got correct result:

1
3
5
7
...

While I use Run button, something wrong and I cant get results:

runfile('/Users/xq/Desktop/data/pycharm_pro/Test_alg/other_test/multi_get_result.py', wdir='/Users/xq/Desktop/data/pycharm_pro/Test_alg/other_test')
Traceback (most recent call last):
  File "/Users/xq/.conda/envs/pycharm_env/lib/python3.8/site-packages/IPython/core/interactiveshell.py", line 3441, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-2-b38026d4d0fe>", line 1, in <module>
    runfile('/Users/xq/Desktop/data/pycharm_pro/Test_alg/other_test/multi_get_result.py', wdir='/Users/xq/Desktop/data/pycharm_pro/Test_alg/other_test')
  File "/Applications/PyCharm.app/Contents/plugins/python/helpers/pydev/_pydev_bundle/pydev_umd.py", line 198, in runfile
    pydev_imports.execfile(filename, global_vars, local_vars)  # execute the script
  File "/Applications/PyCharm.app/Contents/plugins/python/helpers/pydev/_pydev_imps/_pydev_execfile.py", line 18, in execfile
    exec(compile(contents+"\n", file, 'exec'), glob, loc)
  File "/Users/xq/Desktop/data/pycharm_pro/Test_alg/other_test/multi_get_result.py", line 18, in <module>
    print(str(_r.get()))
  File "/Users/xq/.conda/envs/pycharm_env/lib/python3.8/multiprocessing/pool.py", line 771, in get
    raise self._value
  File "/Users/xq/.conda/envs/pycharm_env/lib/python3.8/multiprocessing/pool.py", line 537, in _handle_tasks
    put(task)
  File "/Users/xq/.conda/envs/pycharm_env/lib/python3.8/multiprocessing/connection.py", line 206, in send
    self._send_bytes(_ForkingPickler.dumps(obj))
  File "/Users/xq/.conda/envs/pycharm_env/lib/python3.8/multiprocessing/reduction.py", line 51, in dumps
    cls(buf, protocol).dump(obj)
_pickle.PicklingError: Can't pickle <function add at 0x7fb2204665e0>: attribute lookup add on __main__ failed

Why? Any ideas?