Thank you for raising this question.
Please be informed that we are of this issue and have created a Development card for this so this should be fixed at some point of time. However, I am afraid we do not have an ETA. However, please be advised that it is always best to have weight assigned to all stock items beforehand so that there would be no need to edit orders weights manually. You can use Inventory Import to populate your item with weight in bulk.
At the moment the work around the issue is go to Linnworks.net > Shipping > Packaging Group > click on button under 'Packaging Type' for 'Default' group > insert dimensions of Default Packaging > Save. Once you have dimension saved and try to process order with Default Packaging, the dimensions will be picked up automatically and it will be enough to just enter weight for Save button to be activated. You can also add dimensions to other Packaging types if you want so that orders would have dimensions assigned in case order come with some other packaging rather than just Default.
I hope this was helpful.
Linn Systems Ltd