Sergey pointed out one problem - you've got the FETCH in the wrong place within the loop. What value are you seeing for @txt vs. what value do you think you should be seeing? Also, give this a try:

DECLARE @txt nvarchar(4000)

SELECT 
 @txt = @txt + texto 
FROM
 Universe.dbo.textOdi
WHERE
 certif = 'PM115451'

SELECT @txt

It should produce the same results.

-Mike