Not a group by expression

I am trying as part of my select statement in a view creation to do the logic that if the budget name is like '%Budget% put the value B else if the budget name is like Forecast% but the value F else put A.

I am using the decode command and have the following:

SELECT p.period_name period,cc.segment1 company,cc.segment2 cost_center,cc.segment3 ACCOUNT,apps.gl_flexfields_pkg.get_description_sql (chart_of_accounts_id,3,cc.segment3)account_desc,( NVL (SUM (bal.period_net_dr), 0)- NVL (SUM (bal.period_net_cr), 0))period_balance,( NVL