Can't make a shop

[DimUnion]

I trying to make a chest shop, but when I put a sign on a chest it says "Chest shop can't be used, it may lack a chest" but I clearly but the sign on the chest. What's going on?

 

[EpicAgentJ]

If you are using a trapped chest, it may not work.

Here is a shop example: https://imgur.com/a/78No6
To view yourself, go to /spawn and go over by the crates area. There should be the example there.