Relative specifiers for import statements have to use a file extension: https://nodejs.org/api/esm.html#esm_import_specifiers

Just import { AppService } from './app.service.js'; TypeScript is clever enough to figure out what you want is app.service.ts during compilation.

A workarround could be https://www.npmjs.com/package/tsc-alias as mentioned in https://stackoverflow.com/a/76678279/517319