SQL Server 2008 :: Query Runs Fine Individually But Not Together?

I am able to select the average, max, etc of the variable in a simple select statement but when I do the following, it doesn't work. The reason I am doing the following is because I am calculating the average and such over a 5 min interval, then saving the output as one line. That is why I condense average, max, min, and stdev into one variable which I then output. why it won't run when I have it like this but will run when it is like this?

will run:

SELECT AVG(NacTemp), MAX(NacTemp),MIN(NacTemp), STDEV(NacTemp)
FROM [DATABASE]
WHERE [UTCDeviceTimeStamp] between DATEADD(minute, -5, GETUTCDATE()) and GETUTCDATE()

won't run but I need it to:

DECLARE @now datetime SET @now = GETUTCDATE() --Universal Time
DECLARE @timeint int SET @timeint = '5' --time interval in minutes
DECLARE @time datetimeSET @time = (SELECT MIN([UTCDeviceTimeStamp]) FROM [DATABASE] WHERE [UTCDeviceTimeStamp] BETWEEN DATEADD(minute, -@timeint,@now) AND @now) -- Timestamp data will be saved as
DECLARE @comma varchar(4) SET @comma = ', '

[Code] ....