Hope you are doing great.
You have installed the http-server package locally, rather than globally.
Check the node_modules/.bin/ folder, find the executable and execute that.
If that works try $(npm bin)/http-server if you are running from git bash.
If you use the windows command prompt try node_modules\.bin\http-server.cmd from the same folder where you ran npm install
Please try and let us know if you have any other issue so we can help you out.