Truncate cannot be run with a condition, it will remove all rows from a table (among other things).
You'd need to use a Delete script with a where clause instead, be aware that tblNtLog can have many many rows, and this query could cause a spike in your transaction log.
I'd recommend instead making use of the History cleanup options under Configuration\Server Options. You can specify an interval for eventlogs to be cleaned up. Modify the interval and restart the Lansweeper Server service via services.msc if you'd like to trigger the cleanup immediately. Otherwise it will run at some point in the next 24 hours.