If I'm interpreting the code correctly, you could be able to do the same thing with simply the following:
CREATE PROCEDURE [dbo].[prcLoginv] (
@Username VarChar(50),
@UPassword varChar(50),
@OutRes int OUTPUT)
AS
SET @OutRes =
(SELECT COUNT(*)
FROM [dbo].Log_Users
WHERE Username = @Username
AND [Password] = @UPassword);