Uncaught exception <class 'AttributeError'>... entering post-mortem debugging

I have a piece of python code, when I run it, it did not work out as I expected. So I was trying to debug it using python-debugger package, and got the following message.

uncaught exception <class ‘AttributeError’>… entering post-mortem debugging

The python code is really simple.

if name == ‘main’:

pool = mp.Pool(mp.cpu_count())
print("cpu counts:" + str(mp.cpu_count()))
#with open("./ena_embl-covid19_20210107-0806.txt") as f:
with open("./test.txt") as f:
    # line = f.readline()
    nextLineByte = f.tell()

    for line in iter(f.readline, ''):
      pool.apply_async(processWrapper, args=(nextLineByte,f) )
      nextLineByte = f.tell()

I set the breakpoint at the line of the function apply_async. As the issue is here, because the target function processWrapper is never reached. Any comments are welcome.