When you return the msgReversed variable from the reverseMsg() function, you need to assign it to a new variable in the outside scope.

msgReversed = reverseMsg(plaintext)
(cipher)=encript(msgReversed,k)

I suspect the confusion arises with the following line:

return msgReversed  # Note, the brackets aren't required here

This returns from the function reverseMsg passing out (returning) the variable msgReversed. However, this variable isn't assigned anywhere by default — the name msgReversed is specific to the function. To store a value returned from a function you need to provide a variable name to store it in. Hence the following:

msgReversed = reverseMsg(plaintext)

...will store the value returned from your function in a new variable named msgReversed. You could also name that variable something else:

this_is_another_msgReversed = reverseMsg(plaintext)

When you return the msgReversed variable from the reverseMsg() function, you need to assign it to a new variable in the outside scope.

msgReversed = reverseMsg(plaintext)
(cipher)=encript(msgReversed,k)

I suspect the confusion arises with the following line:

return msgReversed  # Note, the brackets aren't required here

This returns from the function reverseMsg() while passing out the variable msgReversed. However, this variable isn't assigned anywhere by default — the name msgReversed is specific to the function. To store a value returned from a function you need to provide a variable name to store it in. Hence the following:

msgReversed = reverseMsg(plaintext)

...will store the value returned from your function in a new variable named msgReversed. You could also name that variable something else:

this_is_another_msgReversed = reverseMsg(plaintext)

If you don't provide a variable to assign the value to, it is simply lost:

reverseMsg(plaintext)
# No msgReversed variable here