Extracting elements in a list to form a message using for loop

[Tony04]

Hello, I'm trying to make a function that will extract letters from a text document to reveal a secret message. I'm stuck at the at the for loop. It says "list object cannot be interpreted as an integer".

def decodefile():
        fr = open("sentences.txt")
        line = fr.readline()    
        
        codeKey = [[0, 1, 2, 4, 7, 14], [6], [1, 16], [7,8], [14,18]]
        while (line) :
            listLine = [ch for ch in line] 
            for x in range(0, len(listLine), codeKey):
                print(codeKey)

            line = fr.readline()
            
        fr.close
decodefile()

Here is the text if it matters.

They like apples.
I enjoy pears.
Time for breakfast.
Serve the oatmeal please.
Leave a tip for the waiter.