can you create a new folder ('new_mallet') in c:/ drive and unzip the mallet there and try to re-run the code below again
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import os
from gensim.models.wrappers import LdaMallet

os.environ.update({'MALLET_HOME':r'C:/new_mallet/mallet-2.0.8/'})
mallet_path = 'C:/new_mallet/mallet-2.0.8/bin/mallet' # update this path

ldamallet = LdaMallet(mallet_path, corpus=corpus, num_topics=20, id2word=id2word)

result=(ldamallet.show_topics(num_topics=20,num_words=5,formatted=False))
for each in result:
print(each)