Order Not Closed

When you're Assembling and you see this error:

It means that the item you just scanned is on an order that has not been closed. In order to assemble the item you need to close the order. 

Click the X button in the top right corner of the screen to leave Assembly.

Now scan the item again to open the account and click the Close Order Button.

Go back to Assembly and the item will scan like normal.