У вас есть список списков.
Попытка:
word_split1=[['amazon','brand','- ','solimo','premium','almonds',',','250g','by','solimo'],['hersheys','cocoa', 'powder', ',', '225g', 'by', 'hersheys'],['jbl','t450bt','extra','bass','wireless','on-ear','headphones','with','mic','white','by','jbl','and']]
stop_words = set(stopwords.words('english'))
filtered_words=[]
for i in word_split1:
for j in i:
if j not in stop_words:
filtered_words.append(j)
или сгладьте ваш список.
Ex:
from itertools import chain
word_split1=[['amazon','brand','- ','solimo','premium','almonds',',','250g','by','solimo'],['hersheys','cocoa', 'powder', ',', '225g', 'by', 'hersheys'],['jbl','t450bt','extra','bass','wireless','on-ear','headphones','with','mic','white','by','jbl','and']]
stop_words = set(stopwords.words('english'))
filtered_words=[]
for i in chain.from_iterable(word_split1):
if i not in stop_words:
filtered_words.append(i)
или
filtered_words = [i for i in chain.from_iterable(word_split1) if i not in stop_words]