How can I only UPDATE if the user was created?
Taking into account that you need to check the user for a specific database,
you could do something like this?

SET @COMMAND =
'USE '+@DATABASE+' 
 SELECT name FROM sys.sysusers WHERE name = '''+@LOGIN_ACESSO+''''
DECLARE @t TABLE (LOGIN_ACESSO NVARCHAR(255))
INSERT INTO @t 
EXECUTE  (@COMMAND)
IF exists (select LOGIN_ACESSO from @t)
BEGIN
    UPDATE TBL_LOGINPERMISSAO SET CRIADO =1 WHERE [ID]=@ID
END