Visual Studio Team Services API

When I came accross this issue none of the provided answers solved this problem or if it did I didn't like recreating the project. The way I ended up solving the issue:

1. Clicking on the "Connect to Team Projects button" (The plug next to the home button in the Team Explorer tab)
2. Right click the project you are getting this issue on.
3. Click Connect.

I guess the "Remember me" cookie timed out but gave me a generic response for trying to push or pull any code.

Please note that the UI of VSTS has recently been changed. I'm updating my answer to reflect the latest changes but keeping an older copy too!

Update: As of 17th Oct 2018, on Team Services:

1. Account Home - Click on the Azure DevOps icon in the upper left corner.
2. Organization Settings - Click on this link in the lower left corner
3. Process - Click on Process in Boards Group in the list of settings
4. Select your process
5. Select the Work Item Type you want add the new state to
6. Click on States Menu item on the top
7. Click New State button

Adding a Gif below

For those who are still on the older UI , on Team Services...

1. Account Home - Click on the team services icon in the upper left corner.
2. Process - Mouse over the Gear icon and choose Process
3. Choose the process you are working in
4. Click on Product Backlog Item type
5. Click on the States tab (between Layout and Rules)

6. Click the New State link

   • Account Home can be accessed by clicking on the Azure DevOps icon (previously Team Service) in the top-left corner. This page is also the home page of you account and can be accessed by directly jumping using the URL. For example: youraccountname.visualstudio.com/
   • Process can be accessed from the UI as mentioned in steps above or by using the direct URL youraccountname.visualstudio.com/_settings/process

Hope it helps!

Thanks Microsoft for making it available!