pandas._libs.hashtable.PyObjectHashTable.get_item KeyError: 0

I think this error comes from your [i] notation, which is trying to look for the DataFrame index value of 0, which doesn't exist. Try replacing every instance of [i] with .iloc[i].

Also, you should be able to replace the for loop with much more compact, readable, and less error-prone code, especially since you define emotion_map but use it only for output. Try changing the mapping from strings to integers with emotion_map = { 0:6, 1:3, 2:4, 3:5, 4:2, 5:1, 6:0}, then move it to just under filtered_csv = ..., and replace that for loop with

filtered_csv['expression'] = filtered_csv['expression'].replace(emotion_map)

Pandas Dataframe should always be used with iloc as it does not support to indexing with 0. I faced the same issue when there was only single element & I was trying to access that single element by prefixing 0......