It's not clear what you're trying to do. Try following and if it doesn't give result you want, provide more details. Also take a look at ISNULL() function.

SELECT 	loanumber, SUM(amount ) AS Cuota
FROM table1
WHERE amount > 0
GROUP BY loanumber

>Hi friends. I have the following group of command in SQL:
>
>SELECT table1.loanumber, 'CUOTA' =
> CASE
> WHEN sum(table1.amount) is null then 0
> ELSE table1.amount
> END
>FROM table1
>where table1.amount > 0
>having CUOTA >0
>GO
>
>
>But, it send an error message : Invalid colun name 'CUOTA'
>Really I have more command lines in this group but this is the central idea.