There is the error in Kubernetes extension if you are using TFS 2015, you can find the error in Console (Developer tool (F12):

Unable to get property '0' of undefined or null reference
TFS.Admin.ServiceEndpoints.Controls.js (4,4028) 

The issue has been submitted on Github, you can track it:

endpoint connection data not presented when trying to create a service endpoint

Kubernetes extension task is NOT the official Kubernetes task created by Microsoft.

Based on my test, it's not supported on TFS 2015. It only works on TFS 2017+ and VSTS.

In TFS 2015, it can be installed and the Kubernetes appears under the services endpoint list, but nothing happens when select Kubernetes to create a new service endpoint.

So, if you insist on using this task, you can try upgrading to TFS 2017 or later version, or migrating to VSTS.