That error occurs in a SELECT … INTO statement, when your query returns more than one row.
Check your where clause, and make the necessary changes in order to ensure just 1 row is returned.
There is a predefined exception for that error. It is: too_many_rows.
If you know your query could return more than 1 row in some cases, you should put an exception handler, something like this:
SELECT xx into l_xx
when too_many_rows then
' do something