If you get the message:
Tariff Rates incompletely defined for this tariff!
You need to go to Edit->Tariff Rates and make sure all rates have been filled in and click Set at the bottom.

If you get the message:
Dependant calendar time range does not cover all tariff periods
see this wiki

If you get:
Next band does not start where previous band stops
it means that you have enabled banding on multiple line items and the one does not begin where the previous one stopped.
It could also mean that you have multiple band ranges
e.g. a range that goes 0-500-1000-2000-99999999 and another range that goes 500-99999999.
In this case it fails because the system tries to fit the 500-99999999 range between the 500-1000 range.
The system allows for multiple ranges as long as they are linked to different calendars.
So the way around this is to:
-create a calendar that only has one rate (You can use All Off kVA from the templates)
-link the 500-99999999 band to that single tariff rate