The sign-up flow was probably abandoned prematurely. But that is easy to fix. Go to https://my.bikethe.world and log into your account:
When you log in, click create membership:
You now have the option to pick the type of membership you want:
Scroll down and click ‘Pick’:
This will take you to the following page:
Click ‘Proceed to Checkout’ and fill out the following pages as explained here.