ERROR: CASE types character varying and numeric cannot be matched

All the branches of a case expression should return the same datatype. One way to achieve that is to explicitly cast where needed:

,(case when all_loc.country = 'DE' then msc_si.buyer_id::varchar else msc_si.buyer_name end) as "purchasing_group_name_buyer_name"
-- Here -----------------------------------------------^
,(case when all_loc.country = 'DE' then msc_si.planner_code::varchar else mscp.description end) as "mrp_controller_name"
-- And here -----------------------------------------------^