SQL Server 2008 R2 - Shrink TempDB Error

The tempdb recovery is set to simple by default. You can't backup the log of a database in simple recovery. If you have out of control transactions, you'll need to deal with those transactions, then you can try using one of the shrink commands, either shrink database or shrink file. Truncate_only is part of a backup command, not a shrink.

By the way, constantly shrinking the database is a bad practice. If your database is constantly growing out to large sizes, the best practice is to identify the causes and eliminate or mitigate them, not just shrink the database on a nightly basis.

Looks like you have a maintenance plan which uses:

BACKUP LOG [databasename] WITH TRUNCATE_ONLY;
DBCC SHRINKFILE([filename]);

WITH TRUNCATE_ONLY was deprecated in SQL Server 2005 and removed in SQL Server 2008. The same thing can be achieved with

BACKUP LOG [databasename] TO DISK='NUL'
DBCC SHRINKFILE([filename]);

But Grant Fritchey's comment about not constantly shrinking tempdb-files is perfectly relevant.

SELECT sp.session_id 
		, DB_NAME(database_id) AS DBName
		, HOST_NAME AS HostName
		, program_name AS AppName
		, login_name AS LoginName
		, status
		, cpu_time
		, total_elapsed_time AS    ElapsedTime
		, (memory_usage * 8) AS MemoryUsage
		, (user_objects_alloc_page_count * 8) AS AllocatedUserObjectsInKB
		, (user_objects_dealloc_page_count * 8) AS DeallocatedUserObjectsInKB
		, (internal_objects_alloc_page_count * 8) AS InternalObjectAllocObjInKB
		, (internal_objects_dealloc_page_count * 8) AS InternalObjectDeallocInKB
		,CASE is_user_process WHEN 1 THEN 'user session'
				      WHEN 0 THEN 'system session'
				      END SessionType
		, row_count
FROM sys.dm_db_session_space_usage sp
INNER join sys.dm_exec_sessions s
ON s.session_id = sp.session_id