Look at your query:
SELECT (d.ProductID) FROM v_accounts c, t_DormantAccounts d
WHERE (c.ProductID LIKE ('sav9%')
OR c.ProductID LIKE ('cur9%'))
AND c.AccountID=d.AccountID
AND c.OurBranchID=d.OurBranchID
Run that in isolation (SSMS is good for that) and it will return more than one row.
SQL can't work with more than one result where you are using that as a subquery - so you need to either alter that to return a single value:
SELECT TOP 1 (d.ProductID) FROM ...
ORDER BY ...
Or work out a query that will only ever return a single value.
The only other alternative is to look at the whole query that is a part of and work out what you are trying to achieve better, possibly using JOIN.
But we can't do any of that for you: we don't have any access to your DB, the "outer query", or any idea what you are expecting to get as results.