There might be a reason that Typescript is not installed globally, so install it
npm install -g typescript // installs typescript globally
If you want to convert .ts files into .js, do this as per your need
tsc path/file.ts // file.ts will be converted to file.js
tsc // all .ts files will be converted to .js files with in the directory
tsc --watch // converts all .ts files to .js, and watch changes in .ts files