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.