Same as in your other post. Do not use alias column names in the group, because they will not work.
You just have to fully specify the column with table name or table alias and column name. You get the error, because the column PARTNER is part of both tables. So the statement has to know from which table the PARTNER column should be used.
... GROUP BY "T1"."PARTNER, "T1"."BU_GROUP", "T1"."PARTNER_GUID";