WP Super Cache not detected

In Transposh->Dashboard, we are getting this error message at the top of the screen:

We were not able to find a supported in-memory caching engine, installing one can improve performance. Check Transposh FAQs

When I follow the link to the Transposh FAQ
http://transposh.org/faq/#performance
it says “Plugin support: WP-Super-Cache”. So we installed the WP Super Cache plugin.

Naturally, I am confused why Transposh is giving an error that it cannot find a caching engine, when we are using a caching plugin that is listed on the Transposh FAQ page.

Thank you for any advice.

https://wordpress.org/plugins/transposh-translation-filter-for-wordpress/