Operand data type time is invalid for avg operator...?
You can use DateDiff( ms, '00:00:00', e.Duration ) to convert the time into an integer number of milliseconds. Use that for your aggregate, then convert the result back, e.g. Cast( DateAdd( ms, 1234, '00:00:00' ) as Time ).
Improving off of HABO answer:
select top 10Cast(DateAdd( ms,avg(DateDiff( ms, '00:00:00', e.Duration)), '00:00:00' ) as time) as 'avg duration' from TimeTable e
Addition to HABO's and Rafi's answers.
For my case, I had to cast the value of the DATEDIFF to a bigint because my value grew too large and caused an arithmetic overflow error.
CAST(DATEADD( ms,AVG(CAST(DATEDIFF( ms, '00:00:00', ISNULL(e.Duration, '00:00:00')) as bigint)), '00:00:00' ) as TIME) as 'avg_time'