it shows key error

[Vthaore]

19 # Add Constraints
20 for j in loc:
---> 21 model += lpSum([x[(i, j)] for i in loc]) == demand.loc[j,'Demand']
22 for i in loc:
23 model += lpSum([x[(i, j)] for j in loc]) <= lpSum([cap.loc[i,s]*y[(i,s)] * 1000
24 for s in size])
...
3626 # InvalidIndexError. Otherwise we fall through and re-raise
3627 # the TypeError.
3628 self._check_indexing_error(key)

KeyError: 'USA'