Why am I unable to start a job as it keeps saying "unassigned"

Closing or "killing" the Atom app in the background will resolve the issue. Detailed steps are below.

Apple iOS Devices apple

For iOS devices simply double tap the home button when in your main home screen, then swipe up on the Atom app. 

Or if you have a new version of the iPhone without the home button - you just swipe up from the bottom and then swipe each app to close individually.

Android Devices android

Click the recent apps button located normally on the left hand side of the home button and then click close all or swipe up on the Atom app. 
Then re-open the app and perform a sync by dragging the screen down on the jobs lists.
You should see the "depart" button now available for the job.