If you were charged when you expected to be on a free trial, here's the usual reason.
The free trial can only be used once
The most common cause is that you'd already used the 7-day free trial before. The trial is a one-time offer, so starting a membership again charges you right away instead of beginning a new trial.
If you'd like a refund
You can request a full refund from wherever you were charged:
- iPhone: Apple's refund page.
- Android: Google's instructions.