The inventory is created in Datagate before the scan begins. Without a document created, the scanners have nowhere to transmit the data.
From the Inventory menu, click Add.

To be completed:

Click Back — the inventory is automatically saved.
Don't create more than one inventory per day for the same store or warehouse. If you accidentally created more than one, delete or move the date of the wrong ones before you start scanning.
In the ICS scan app, press Sync to retrieve the newly created inventory on the scanner.