Bug:
If you cheat strength or charm at the beginning, increasing those values in the game concatenates, instead of adding, to the value. So if you give yourself '100' strength, and then train, you will have '1001' strength, then '10011' strength, not '101', then '102' as expected.
Not sure about Twine specifically, but the javascript reason is that it's trying to add a string with an integer instead of two integers. The value you put into the cheat menu needs to be converted to an integer first before actually being set so that adding to it works later.