It looks like the flatdemandmonths -> ur_dc_flat_mat converter works on accident.

The variable flat_demand derived from flatdemandmonths references periods, not tiers. Unfortunately the converter swaps the concept of tiers for periods in this line and accumulates a series of tiers, but calls them periods on the same line.

If there it more than one tier, it then emits an incorrect error message stating that it does not support more than one period per month. flat demand rate structure does not support multiple periods per month

I wonder if ssc might support more than one tier per month if given a correct data-structure.

Regardless - Variable names and error messages should be corrected.