It seems that you didn't login to the Kasa with a cloud account or the HS100 lost connection from your router.
You can first create a cloud account and login to Kasa with it.
http://www.tp-link.com/en/faq-948.html
If the issue happens again, pay attention to the wifi light on HS100. It should be solid green, otherwise it lose connection from your router.
Move it closer to the router will help.

I have same issues like Shield101. The green LED is always switched on, but Alexa says: "TP-link reagiert nicht" (german). Sometimes after 5 min the light will be switch on. Sometimes nothing happened :-/ This error still exists since 3 month I think.

Make sure kasa can control the device locally and remotely first.
Move Amazon Echo close to the plug and do the test again.