environment variables.js cannot be read

I'm trying to add JWT Auth Token in my react js application for authorization. For this I'm following this example. When I run the code I get the following error:

Error: Config file /config/custom-environment-variables.js cannot be read. Error code is: undefined. Error message is: FileSystem.readFileSync is not a function

Here is the output from my console:

I have seen this question, but it didn't helped me.