score:9
You or your adviser misunderstood something. You will not need to show the credit/debit card as proof of ID while purchasing the Bahncard online (how are you going to show a credit card online?)
You will need to prove your ID later - if you buy a train ticket online and the ticket controller asks you to present an identity document. This document must not be a credit/debit card. It may be almost any kind of ID and even your Bahncard itself (see answer to this question).
But there is another issue - if you're under 18 (which I suppose) you cannot buy a Bahncard at all! However you can buy a "Jugend Bahncard" (good news - it costs only 10 Euro) but you need a written permission from your parents for that.