have been using taxjar.com with... ok results so far - they seem to only provide a sandbox/playground for 'pro' accounts ("call for pricing" - ugh).
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?