'List' object has no attribute 'upper'

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Python

Jupyter

def removeDigits(str):
    return str.translate({ord(i): None for i in '0123456789'})

def fileinput():
    with open('constant.txt') as f:
        lines = f.readlines()
    
    print('Initial string: ', lines)
    res = list(map(removeDigits, lines))
    print('Final string: ', res)
    
    print('Make string upper or lower?')
    choice = input()
    if choice.upper() == 'UPPER':
        print(res.upper())
        
    elif choice.lower() == 'lower':
        print(res.lower())
    else:
        print('An error has occured')
    

fileinput()

What I have tried:

Initial string:  ['Geeks123for127Geeks']
Final string:  ['GeeksforGeeks']
Make string upper or lower?
UPPER

This is the current output. However, after specifying whether I want upper or lower the program crashes saying there is an attribute error. How would I fix this?