Unless your sign is owned by the server, where items are created out of thin air, you will need to connect your sign to a chest.
To connect a chest, type /duckshop link
.
Then follow the on-screen instructions to complete the link.
Links are saved automatically, so you only need to connect it once.
Chest linking uses the same permissions as placing signs, so to link a chest, you must have the appropriate create
permission.