set iin_userid = (select emp.empid
from employee emp, itinventoryr1 it
where ltrim(rtrim(it.iin_username)) in (emp.empname))
Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.
I guess you need something like this:
UPDATE R1 SET iin_userid = emp.empid FROM itinventoryr1 R1 INNER JOIN employee emp ON ltrim(rtrim(R1.iin_username)) = emp.empname
Join tables, and update one of them.