That message is something to do with either your address or the buyers address code that appears on the label.

Erm...have to think what l did to fix it...when you have the label screen open, right up near the top of the screen is your address (it says Address / Group Code). If you click on the little arrow thingy to expand your address, is there an issue there anywhere? Full stop after your town name, wrong post code...something like that.

If that is all correct, then go down to the buyer's address and check that.

One of the two is wrong.