The error means exactly what it says: your subquery returned more than one value.
Your code is effectively
SELECT a_column * (a_subquery) Amount FROM Mytable
return more than one value - which means you are asking SQL to multiply NumUnit by several values and it won't do that.
You need to look at your subquery closely - and I'd suggest running it on it's own to see what it does return - and sort it out so that it does return just one value.
Unfortunately, we can't do that: we don't have access to your DB, and we have no idea what you are trying to get NumUnit multiplied by!