Summary:
Error Cannot book Time off against this Time off type, might occur when you try to save, submit, reopen or unsubmit a timesheet.

Cause:
It is possible that a time off may have been booked or added on the users timesheet, and that time off type is disabled in Web TimeSheet.
If this time off is disabled at the global level, under Administration -> Time Off Setup -> 'Time Off Types', or in the users profile, then this error will occur while trying to save, submit, reopen or unsubmit a timesheet.

Resolution:
Enable the timeoff type and then you can save, submit or reopen the timesheet. If you don't want that time off, then remove it and save / submit the timesheet.