When trying to create a report for postponed tasks, an error message appears and I cannot save the work.

You cannot create a report with a task date earlier than the date set in the Postpone  report. If it were possible, it would be unclear whether the next due date is the set date in postpone report or calculated date according to the interval. If you encounter this error, please delete the Postpone report and then create a work report again. Deleting the Postpone report will temporarily record the item as Outstanding, but it will be excluded from Outstanding when a Work Report is created.