SQL & PL/SQL

I m using this but giving error "Encounter the Symbol "Where" when expecting one of the following.

Code is :-

insert into dum (cost_no, c_size, cmt1, cmt2, cmt3)
values (:sizecost.cost_no, :sizecost.c_size, :bottomcost.cmt1, :bottomcost.cmt2, :bottomcost.cmt3)
where :sizecost.cost_no := :costmain.cost_no;