Very Urgent Help

Hello,

I am trying to run the following

if (select REQUEST from TEAM3B_PULL_TOTAL_TST) != (select REQUEST from REQUEST_BUS_REQ_TST)
begin
Insert into REQUEST_BUS_REQ_TST (b.request,b.business_req_id,b.bus_test,b.test_cas e_id,b.test_case_source,b.test_case_descr)
select a.request,a.business_req_id,a.bus_test,a.test_case _id,a.test_case_source,a.test_case as test_case_description
from TEAM3B_PULL_TOTAL_TST as A JOIN REQUEST_BUS_REQ_TST AS B ON A.REQUEST = B.REQUEST
Order by b.REQUEST,b.BUSINESS_REQ_ID,b.TEST_CASE_ID
end

and I keep getting the error message of
Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.

I have tried using not in with this statement and it also gives me the same error. I need to say if request in table one does not equal request in table two then insert values into table one. Else insert the values into another table(table 3).

Any help would be greatly appreicated.!!