df_first_melt[cols] 이부분 에러가 계속 나요..

안녕하세요 다 잘 되는데 이상하게

df_first_melt[cols] 이부분에서 

---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-119-8f828eeadc48> in <module>
----> 1 df_first_melt[cols]

~/opt/anaconda3/lib/python3.7/site-packages/pandas/core/frame.py in __getitem__(self, key)
   2984             if is_iterator(key):
   2985                 key = list(key)
-> 2986             indexer = self.loc._convert_to_indexer(key, axis=1, raise_missing=True)
   2987 
   2988         # take() does not accept boolean indexers

~/opt/anaconda3/lib/python3.7/site-packages/pandas/core/indexing.py in _convert_to_indexer(self, obj, axis, is_setter, raise_missing)
   1283                 # When setting, missing keys are not allowed, even with .loc:
   1284                 kwargs = {"raise_missing": True if is_setter else raise_missing}
-> 1285                 return self._get_listlike_indexer(obj, axis, **kwargs)[1]
   1286         else:
   1287             try:

~/opt/anaconda3/lib/python3.7/site-packages/pandas/core/indexing.py in _get_listlike_indexer(self, key, axis, raise_missing)
   1090 
   1091         self._validate_read_indexer(
-> 1092             keyarr, indexer, o._get_axis_number(axis), raise_missing=raise_missing
   1093         )
   1094         return keyarr, indexer

~/opt/anaconda3/lib/python3.7/site-packages/pandas/core/indexing.py in _validate_read_indexer(self, key, indexer, axis, raise_missing)
   1183             if not (self.name == "loc" and not raise_missing):
   1184                 not_found = list(set(key) - set(ax))
-> 1185                 raise KeyError("{} not in index".format(not_found))
   1186 
   1187             # we skip the warning on Categorical/Interval

KeyError: "['연도'] not in index"


이런 에러가 나는데 왜 그럴까요..