Like the title says. Seems like you can only use production accounts with the Square app
Currently the ability to test in sandbox with the Square app isn’t currently available. You will need to use production to test with the app. We’re constantly working to improve our features based on feedback like this, so I’ll be sure to share your request to the API product team. 
Great, thanks Bryan!
Hi Bryan, is the only (best) way to test Sandbox account using Postman or similar API testing tool?
Yeah, Postman is great and we also have our API Explorer that you can use to build requests and run them in sandbox.
Greetings, just wondering if this is still the case. Was hoping to test our new webstore in sandbox mode.
Yes, at this time there isn’t any testing environment with the Square app at this time.