If your application was already approved and you've received the confirmation email, it may just be a delay while the benefits finish activating on your account.

I'd first sign out and back into GitHub, then check your Student Developer Pack page again. If the 72-hour waiting period has already passed and the benefits still aren't active, I'd recommend contacting GitHub Education Support with your approval email and explaining that the account still shows the 72-hour waiting message.

Hopefully it gets activated soon!