Editing Dictionaries

[j.crater]

                Pair = 'A'
        elif i == 1:
                Pair = 'E'
        elif i == 2:
                Pair = 'U'

Here you assign a string to Pair and not the dictionary you want to work with. This means your code really does:

'A'['Bi'] = Pair + 'b'  # when it should be A['Bi'] = Pair + 'b'

So instead just remove the parentheses from A E and U.