This should do the trick (assumes seat_no is the same data type in both tables):
select seat_no.seat_no
FROM Seat_No
where seat_no.seat_no NOT IN
(
SELECT Pasenger_Detail.Seat_No
FROM Pasenger_Detail
WHERE (((Pasenger_Detail.Group_ID)=forms!Pasenger_Detail!Group_ID)
And ((Pasenger_Detail.BR_ID)=forms!Pasenger_Detail!BR_ID))
)