I can't buy a shipping label

Work down this list. One of them is almost always the answer.

1. A backup card on file. Settings, then Backup card. Without one, shipping is blocked before a rate is even fetched.
2. A complete ship-from address. Settings, then Ship-from address. Street and ZIP are both required: postage cannot be priced from a partial address.
3. A box size and weight on the lot. If the listing has none, the ship screen asks for them at label time. Enter the real ones.
4. An order that has not already been labelled. If the order shows a label, the second attempt is refused on purpose.

If all four are in place and the label still fails, the carrier rate lookup itself failed, usually a box size or weight the carrier will not accept. Correct the numbers on the ship screen and try again.

A message about insufficient funds, or a redirect to a carrier or postage site, is not about your balance. Report it from the order and it will be looked at. Nothing is charged to you in that state.