Colleagues, Success at last at overcoming the dreaded Sendle “Something went wrong and we weren't able to generate the label”!!  In addition to the receiver’s address being valid for Sendle, it would appear that your address as the sender also has to be valid AND deliverable to by Sendle (I assume in case they need to return the item as undeliverable).  I was using a PO Box as origin for sending from and receiving items.  I have updated my Primary Address from the PO Box to my physical home address (My eBay > Account > Addresses > Return Address). I had to do this for all 5 of the addresses I had in eBay (Registration Address, Postage Address, Dispatch Address, Return Address and Payment and Pickup).  I was then able to generate a Sendle Label.  I am now in the process of switching all my addresses back until I next need to generate another Sendle Label.  Thanks to sqleonhart who tipped us off about the Dispatch Address.