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'.
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?
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?