Nope. I've logged in. Registered. I'm a
paying customer, using it in production, and there's no visible way to generate a sandbox key, except 'go to plus'.
From their docs: https://developers.taxjar.com/api/reference/#sandbox-environ...
* Sandbox Environment
TaxJar provides a sandbox environment for automated testing and development on all TaxJar Plus plans. After generating a sandbox API token, point your API client to the sandbox environment...
There is no sandbox API available without 'plus'. Perhaps you had an earlier version and are grandfathered in?