string indices must be integers

[TomToad]

data is the dictionary. When you do for element in data, element takes on the value of the key, first iteration, it is 'sentence', second iteration, it is 'tags' Get rid of the for loop and it will work.

import json
data = json.loads('{"sentence": ["what", "is", "a", "transistor"],"tags": ["WH", "AUX", "DT", "NN"]}')
print(type(data))
print(data["sentence"])
 
sents = []
tags = []
 

sents.append(data['sentence'])
tags.append(data['tags'])