Your Bend account and your subscription are two separate things. Here's what each one is and how they fit together.
Your Bend account
Your account is a login you create with an email address and a password. It saves your progress and statistics, and it lets you use Bend on more than one device. You create it from the Profile tab by tapping Create Account.
Creating an account is what lets you log in. If you've never made one, trying to log in shows "User not found" because there's no account to sign in to yet.
Your subscription
Your subscription is what you paid for. It's linked to your Play Store account on Android, or your Apple ID on iPhone, and it's handled by Google or Apple rather than by Bend. Because of that, you manage or cancel your subscription through that same store.
How they work together
You can have a subscription without ever creating a Bend account, which is why the two can feel disconnected. When you do create an account and log in, your subscription syncs to it automatically, so your access follows you to any device you sign in on.