Unable to add a value to test configuration variable "Operating System"

I'm using Visual Studio Team Services (VSTS) along with Microsoft Test Manager (MTM) 2015. When I attempt to add a value to the existing "Operating System" configuration variable in MTM, I get an error saying "The configuration variable cannot be saved: Values in test variable 3 are in use and cannot be deleted".

When I try to do the same thing through the web using VSTS's test hub I get an error with the same message.

I'm not trying to delete any variables, I'm simply trying to add one. What am I doing wrong?

Try to refer to these steps to deal with this issue:

1. Send request (Get https://[account].visualstudio.com/[team project]/_apis/test/Variables/3?api-version=4.0-preview.1)
2. Check the name and values
3. Check this configuration variable values of all test configuration whether they are matched. If not, change it
4. Add a new variable value to a configuration variable

You also can delete related (step 2, variable name) variable in all test configurations, then add new variable value to that configuration variable, after that add this configuration variable to test configurations again.