What is your CALENDAR_WEEK column?
Because your CASE can return a double (QUANTITY), a xxxxx (CALENDAR_WEEK) or a number (the ELSE).
Calendar_Week is a varchar column
Your CASE statement "return values" should be of a consistent data type. In your case, you're returning a VARCHAR, NUMBER & a DOUBLE.
I suggest that you convert your "return values" to a consistent data type, in your case, convert quantity by doing this:
CAST(Planner WorkBench"."DW_SUPPLY_DEMAND"."QUANTITY" AS CHAR)
and replace 0 with '0'.