then what must be used ?

Not a kata issue, your function is returning None or not returning and that's the default return value for Python. You can see in the kata description that False is the expected value for those input values.

Also take note of the edge case - if both strings are empty then you should return false/False.

Read the error message:

ValueError: invalid literal for int() with base 10: 'n'

You can't convert a letter to an int, that's what it is about.

It's your duty to verify that n is a valid positive integer number.

You code only passes 4 tests (there are more), not a kata issue.

Try this one in sample tests:

test.assert_equals(f('n'), None)