I encountered a similar issue this is what I did. I hope you will find it helpful. I appreciated everyone's support.

Step1: // app.module.ts

  imports: [
    UsersModule,
    GraphQLModule.forRoot({
      // autoSchemaFile: true,    did not work!
      autoSchemaFile: join(process.cwd(), 'src/schema.gql'),
      // schema.gql will automatically be created
      debug: true,
      playground: true,
    }),
  ],
  providers: [AppResolver],   // all resolvers & service should be in providers

step 2:

Make sure you have one at least query bcz Mutation is not enough if you don't GraphQL playground will not start. Read more here