Nope, still getting the same error a few months on - seems nobody cares!
Foxnetsoft has a plug-in for this that actually works much better than the nopcommerce one. I suggest to use that.
What Square API version are you using?
It appears to be the same in 4.2. To get this working, I had to add the location workaround that is mentioned on Github under issues. This passes the location ID to the API. Also, the base url in the plugin will only work with Production, sandbox is a different endpoint.
Also, the admin interface doesnt' appear to save the data correctly so update it by changing setting or directly in the DB.
Not the best, but it can be made to work with a few tweaks.