Why can't I move the task deadline into the future?

It is possible to postpone the deadline for any of the tasks. Just click on the task detail and click on the Edit button to postpone the task deadline. If you have grey data and the deadline cannot be pushed further into the future, make sure that you do not try to enter the deadline of the task after the deadline of the whole checklist of which the task is a part.

You can check this in the Onboarding and Tasks / In progress section where you can find the required checklist. Click on it and go to the Detail tab. Here you can edit the checklist deadline and then the task deadline.