According to the email you received, it seems something went wrong when checking out at Amsterdam Bijlmer ArenA. Because of that, the fare you should pay cannot be determined, and you are charged a correction fee of 20 euros instead.

You should be able to fix this yourself by going to Mijn NS (the website, not the app), clicking My travels > Journey history, finding the corresponding check-in and clicking a link that should enable you to manually specify you checked out at Amsterdam Bijlmer ArenA.

If you have any problems with that, you should contact customer service directly and have them fix this for you. This can be done by phone; they are available 24/7 at 030-751 51 55. You may also contact them by chat (click the link Chat with NS on this page) or by sending a message on X (formerly Twitter) or Facebook.