'truncate_only' is not a recognized BACKUP option.

Error:

'truncate_only' is not a recognized BACKUP option.

Reason:

This error will occur, if you run the 'Backup Log' query in 'SQL Server 2008'. Because, SQL Server 2008 will not support this command. 

Your Query: backup log WSS_Content_5001 with truncate_only

Solution:

So, you have to run query in the below format.

Use DBName

GO

Alter Database DBName Set Recovery Simple

GO

Alter Database DBName Set Recovery Full

GO

DBCC SHRINKFILE ('Log Name', 1) 

That's it...