You can sign up for the Stryd Membership within the Stryd app on your mobile device. The subscription is then managed through the Apple or Google app stores (you must cancel your Stryd Membership there if you wish to do so).
If you are being charged for the Stryd Membership, but you do not have access to the membership features, there was a glitch that you can resolve yourself. You must use the phone to restore your membership, you cannot use PowerCenter on your computer.
Open up the Stryd app on your phone
Go to settings > User Profile. You will see a Subscription section. Click on whatever link shown there, it will take you to a new screen (you will not be charged at this point)
You will now see a Restore Subscription option. Example below:
