ind
0   ['C']
1   ['C']
2   ['C']
3   ['C']
4   ['E']
5   ['E']

from itertools import chain

df = pd.DataFrame({'ind': [['C'], ['C'], ['C'], ['C'], ['E'], ['E']]})

res = ''.join(list(chain.from_iterable(df['ind'].values)))

print(res)

CCCCEE