The battery page is showing as incomplete but I have a battery?

This page can only generate with the Easy PV or MCS self-consumption method selected in the Financial task.

Navigate to the Financial task. Click the Financial inputs in the top right, then make sure MCS or Easy PV is selected:

If you have opted to use a custom self-consumption percentage, Easy PV doesn't have the information required to determine the battery utilisation, which is why this page is hidden.