When you type
npm run server.js
npm tries to find the entry named server.js in the scripts section of your package.json file (see npm docs on scripts for details).
package.json, simplified, describes your app's dependencies and environment.
As you don't have such file, npm fails with ENOENT, which is an abbreviation for Error NO ENTry.
Basically, in your case, you just want to node server.js to run your script.
Alternatively, you can try the following command to resolve the issue:
npm install -g expo-cli --force
This command has been reported to work for some users.