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 -----------------------------------------------^

SELECT "table","schema",
CASE 
WHEN  "size" <= 1024 Then SIZE::varchar || 'MB'
WHEN  "size" > 1024 AND "size"  < 1000000 Then ("SIZE"/1024)::varchar || 'GB'
WHEN  "size" > 1000000 THEN ("SIZE"/1024)/1024::varchar || 'TB'
END size
FROM SVV_TABLE_INFO 
order by 1;