I Am Getting An Error Message For This Query

SELECT T1.lnumber AS LOT_Number,(T2.time_out - T1.time_in) AS Duration,
((DATEPART (hour, (T2.time_out-T1.time_in)) *60) + DATEPART (minute, (T2.time_out-T1.time_in))) AS Minutes, COUNT(DISTINCT T2.test_desc) AS Number_of_Process

FROM Results AS T1 JOIN Results AS T2 ON T1.lnumber = T2.lnumber

WHERE T2.test_desc = 'Shipping' AND T1.test_desc = 'Receiving'


I have an error message that says:

Msg 8120, Level 16, State 1, Line 3
Column 'Results.lnumber' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.


If I take this line out: COUNT(DISTINCT T2.test_desc) AS Number_of_Process
The query runs ok.

Also if I run [COUNT(DISTINCT T2.test_desc) AS Number_of_Process] in:

SELECT COUNT(DISTINCT T2.test_desc) AS Number_of_Process, T2.lnumber AS Lot_number
FROM Results AS T1 JOIN Results AS T2 ON T1.lnumber = T2.lnumber
GROUP BY T2.lnumber

It will also give me result.
Please help....
Thanks for your time