Obviously there is more than one distinct dept_payin_no - that is what you are being told with "Exact fetch returns more than one row."

So your "I am trying ot retrive only one value for a column which as same values for all records" won't work - you have more than one value that covers all records. Go into SQL*Plus and do select distinct dept_payin_no and you should see more than one value for dept_payin_no.

If you are expecting only one value, your data is broken. Find out how and why that happened, take steps to prevent it in the future, then work on the trigger. That's one solution. There are others, but with your expectation that only value is to be returned, you have other problems to worry about first.