There's no FOR EACH ROW syntax in SQL Server triggers. What you have instead are two tables called inserted and deleted that contain ALL rows affected by the transaction. Something along these lines should point you in the right direction, obviously you'll need to do your own verification of this:

CREATE

TRIGGER transactiontrigger ON ORDERINVOICE

AFTER INSERT

AS

BEGIN

UPDATE A_ACCOUNT

SET A_ACCOUNT.AC_RUNNING_BLC = ( A_ACCOUNT.AC_RUNNING_BLC

- inserted.NETAMMOUNT

+ inserted.COMMISION

- inserted.TDS )

FROM A_ACCOUNT

INNER JOIN INSERTED ON A_ACCOUNT.A_ID = inserted.A_ID;

END;