Uncaught TypeError: Reflect.defineMetadata is not a function on Angular 7 Production Build

Make sure that you have reflect-metadata installed. Also try adding this line to your polyfills.ts file:

import 'reflect-metadata'

OR

import 'core-js/es7/reflect';

Ideally, this(the second import statement) is already a part of the polyfills.ts file.

That should make it work.