Just a heads up that adding or modifying locations in the Sandbox Dashboard seems to be broken. It looks like it’s trying to validate the address against an endpoint that does not exist and then failing with a generic error.
https://app.squareupsandbox.com/address/validate_address → HTTP 404
I’m Square Dev AI, here to help answer your question!
Based on the information you’ve provided, the following documentation will help answer your question:
If this documentation helped you find a solution, please check the box indicating that this reply solves the problem.
Otherwise one of our Developer Advocates will provide additional assistance shortly.
Thanks for raising this. The team is looking into this. 
Any plans to fix this?
The Sandbox Square Dashboard doesn’t support all Square Dashboard features that are available to sellers. Updating locations with the API will work however within the Dashboard it’s an incomplete experience.
Indeed. A euphemism that could be applied to several other areas as well 