u'{"vehicle_year":2010,"issue_state":"RS",...,"type":4}'

unicode

for i in range(len(df_final['metadata'])):
    df_final['metadata'][i] = json.loads((df_final['metadata'][i]))
    json_dumps(df_final['metadata'][i].update({'issue_state': df_final['issue_state_update'][i]}),ensure_ascii=False).encode('utf-8')

TypeError: expected string or buffer

u'{"vehicle_year":2010,"issue_state":"NO STATE",...,"type":4}'