This issue is normally caused because the advertised task sequence to the client has no Boot Image associated with it.
To fix the problem you need to associate a boot image with the task sequence by following the steps below.
· Right click on the affected Task Sequence and choose Properties.
· Click on the Advanced Tab.
· Make sure that the option "Use a boot image:" is checked and that a Boot Image is selected.
Once you do this, Operating System Deployment should work as expected.