What I have the erroe: TypeError: string indices must be integers? How can I solve this if I want bt to be outside the function back_trans?

Question

def main():

pep = \'MEGNCS\'
newseqs = []
bt= {\'I\': [\'ATA\', \'ATC\', \'ATT\'], \'M\': [\'ATG\'], \'T\': [\'ACA\', \'ACC\', \'ACG\', \'ACT\'], \'N\': [\'AA