When printing a shipping label for an order, the label is not printing. You can see red cross in the Flags section for the order.
The shipping module has generated an error. The error itself is most likely has been returned by the courier's API or during the validation. In simple terms there is a problem with the order, i.e. weight is required, unlinked items or an invalid shipping service code
Move your mouse over the red cross, a popup message will be displayed showing the error. In most cases this will give you an idea what is causing the problem. Search knowledge base for similar errors to find out the exact resultion for specific shipping label issues.