Note:
  Your configurations should be based on your Debug.xconfig or Release.xcconfig
  file, not the Pods-Runner.xcconfigs. You can check this by expanding the configuration names.

To match the free flavor, add -free 
at the end of each new configuration name.

Change the free scheme to match the build configurations already created.
    
      In the Runner project, click Manage Schemes… and a pop up window opens.
      Double click the free scheme. In the next step 
(as shown in the screenshot), you’ll modify each scheme 
to match its free build configuration:

Using flavors in iOS

Now that you’ve set up your free flavor, 
you can, for example, add different product bundle identifiers per flavor. 
A bundle identifier uniquely identifies your application. 
In this example, we set the Debug-free value to equal 
com.flavor-test.free.