This functionality is only available in Linnworks.net.
However, you can still print labels from Linnworks desktop application and 2D barcode will appear in Tracking Number field in order details. All you need to do is add Royal Mail integration to Linnworks.net and add the services that you use. If you wish to keep the same Linnworks postal service name, then services should be removed from your desktop integration. Also you can create new service names if you wish.
Then you can print labels from desktop application using your Linnworks.net integration. Please note that you will need to file the manifest from Linnworks.net (Shipping > Shipping Manifest > File).
2D barcode will only be generated for untracked services. For tracked service Linnworks will still generate a tracking number.
Is the Royal Mail 2D barcode integration available on LW desktop?