Why is the battery page is showing as incomplete but I have a battery in the project?

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

To fix this, follow these steps:

1.  Navigate to the Financial task
2. Click the Financial inputs in the top right
3. Then make sure MCS or Easy PV is selected:

This is because Easy PV needs to model how much of the consumption is coming from solar directly, from the battery or from the grid in order to generate that page. When you select 'Enter value', you are giving a flat self-consumption rate that includes all solar self-consumption, regardless of whether it's used to charge the battery before it's consumed directly.