Use of sum and group by in SQL

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SQL-Server-2012

Hi guy's
hope every one is fine.
i have one question from every one please give me the solution of my problem
i try to use sum function, inner join and group by clause in same time but its not working
please any one tell me how can i do this
i try this code but it show error

What I have tried:

    select s.[ProModelNo]+' '+p.ConditionPro+' '+p.ItemType    as 'Item Description',
     s.[ComName]       as'ComName',
 sum(s.AvailableQty)   as 'Avil'
from [dbo].[Stock] s
inner join [Products] p
on p.ModelNo=s.ProModelNo
group by  [ProModelNo],[ComName]
having  sum(s.AvailableQty)=0 or sum(s.AvailableQty)=1