If you are scanning items in Assembly and you get the "Item Not On An Open Order".

Scan the item in the main search box. This will open the customers account with the last order the item was on display at the top of Clothes Tracker. The item will have a green banner saying "Current".

If there's an existing order in process for that customer click the order number of that in process order then click Add Item and scan the item that gave you the error.

If the item belongs to a hotel customer or an account where you process many orders at a time for different hotel staff, guests, police officers, company employees, etc., Then just put the item on a new order with the same extra information as the last order it had, or look for an order in process with that same extra information and add it to that order.