Add -f to your package.json file, Ex: env-cmd -f ./config/dev.env nodemon src/index.js

Add -r since .env is a file and we want to read the content of it.

I also changed the default paths and added some of my paths to work smoothly on the localhost.

If you need to add the local paths then follow {node-modules -> env-cmd -> dist -> get-env-vars}  in here you will get an array of default paths, add your path 

"dev": "env-cmd -r ./src/config nodemon src/index.js" do this way