Where can I see whether my reward is waiting, available or applied?

Check You → Invite a friend:

Waiting: the conditions for the reward have not all been confirmed. Available to redeem: an App Store reward is ready for you to claim. It has not yet extended your subscription. Applied: the billing service has confirmed that the reward was used. Not eligible: this invitation cannot produce a reward under the programme's rules.

If the result looks wrong, use Contact support from Help and include the state you see and your billing service. We can check your reward without asking you to send your friend's private account information.