Amazon made a change to their security such that if you go to the user profile page (click on your name at the top of the KDP home screen), you must first provide a one-time password. BookTrakr goes there in order to determine which KDP account you are logged into (some users have more than one). Having to work with this one-time password can be annoying for you.
BookTrakr now uses information that it can get from the KDP Bookshelf page to infer your KDP account ID. Sometimes it will be unable to do so, in which case you will still have to provide the one-time password, but usually the Button can continue its processing without you having to do anything extra.
Note that with a new Button installation you will get the one-time password dialog (this includes when you install the Button on a different computer). And, of course, if you have turned on two-factor authentication for Amazon, you will be asked for the one-time password every time the Button runs.