Return label not generated for post office return

As BobThe Fisherman suggests, examine the shipping (to you) label VERY closely:

In most cases, the return shipping label is physically UNDER a peel-off layer.

When the label to you is carefully peeled off, the return label will be revealed underneath.

EUNU wrote:

There's no pickup location nearby, so I'd like to return it via the post office, but the label hasn't been generated yet. What should I do?

Return what?

If you received a return box the return label is likely under the label addressed to you.