That's the exact message that comes up when you have the sending address set as a PO Box. Just change the sender's address to a street address to avoid the issue.

Just a word of advice (based on my personal experience)... the best way to use Sendle is to log in to your Dashboard. From the main menu, go to integrations and then select "Fetch Orders"  (obvious orange button). Locate the item you wish to send and select "Send parcel" (orange and white button at the bottom of each unsent item details). This will open the "Send Parcel" page with all relevant details pre-filled. You just need to enter weight and dimensions (or just select satchel if you are using a Sendle satchel). The price will show at the bottom of the page. Once you order the Sendle label and confirm the order, the tracking details will automatically appear on eBay.

Please note: doing it this way is far easier and a bucket load cheaper, too. The only drawback is that the labels take up more than half a page and print on all 4 labels when using a 4-up label sheet, whereas the eBay generated label fits neatly onto the 4-up label sheet without any problems.

I hope that helps.