Lot Costing errors with UOM's

We are a process manufacturing org using lot costing method who implemented ASCP recently. We are setting up the routings for new products that have a lot of lead times and testing them in lower instance. So we are trying to build routings with resources that have uom of period class. As a part of it, we have defined custom uoms like MTH (month), WK(week) and using it. The routing has 2 operations - 1 st operation is associated with OSP resource whose uom is G (Grams) and the second operation is associated with resource who uom is associated to MTH.