Creating 2D barcodes for box content requires you to have an open FBA shipment. Keep in mind that a shipping plan is different than an actual FBA shipment. Here's the difference between the two. If you have a shipping plan, please convert to an FBA shipment before proceeding to box content.
1) Here's a video on the process.
2. When finishing up the shipment on Seller Central, please select the 2D barcode option.