Is your station set to the wrong fulfillment location?
You must have a station set in order to use Springboard Retail's order fulfillment system.
Once you have selected the station, you are only allowed to fulfill orders that have been distributed to the store/warehouse where that station is located. (This is to prevent other stores from accidentally fulfilling the wrong store's orders.)
On the fulfillment page, when you are creating the invoice, you will see a section that lists the lines from the order that have been distributed to your location.
In the screenshot above, the location I am physically located in is the one that is displayed next to the Order Lines text (e.g. "Boston") If no items appear in this section, it means that no lines from the order have been distributed to Boston.
Also, if the location you see doesn't match the location you're in, chances are, you have set the wrong station. You can change your station in the top right menu by hovering over your name, then selecting the "Station (Change)" option from the menu.