You are inside the backend folder when running the command right?
Try adding nodemon to the dev dependencies and use it as a script inside package.json (instead of running it as a global install).
Not sure if any ambiguity exists (say a folder /server and a file server.js ) and you give nodemon the file extension with the run command if it would pick the right one or not. I have never really used it as a global install (I think it is best to avoid globals).