GraphQLError: Query root type must be provided

I'm using NestJS, TypeORM and GraphQL for my backend API. I'm getting the following error:

GraphQLError [Object]: Query root type must be provided.
      at SchemaValidationContext.reportError (/home/wise-introvert/Container/Projects/the-notebook/app/server/node_modules/graphql/type/validate.js:88:19)
      at validateRootTypes (/home/wise-introvert/Container/Projects/the-notebook/app/server/node_modules/graphql/type/validate.js:107:13)
      at validateSchema (/home/wise-introvert/Container/Projects/the-notebook/app/server/node_modules/graphql/type/validate.js:52:3)
      at graphqlImpl (/home/wise-introvert/Container/Projects/the-notebook/app/server/node_modules/graphql/graphql.js:79:62)
      at /home/wise-introvert/Container/Projects/the-notebook/app/server/node_modules/graphql/graphql.js:28:59
      at new Promise (<anonymous>)
      at Object.graphql (/home/wise-introvert/Container/Projects/the-notebook/app/server/node_modules/graphql/graphql.js:26:10)
      at GraphQLSchemaFactory.<anonymous> (/home/wise-introvert/Container/Projects/the-notebook/app/server/node_modules/@nestjs/graphql/dist/schema-builder/graphql-schema.factory.js:49:52)
      at Generator.next (<anonymous>)
      at /home/wise-introvert/Container/Projects/the-notebook/app/server/node_modules/tslib/tslib.js:114:75

This is what my file structure and code looks like:

Can someone please help me. My repo: https://github.com/wise-introvert/nestjs-graphql-api.git

@Resolver()
export class FooResolver {

  @Query(() => String)
  sayHello(): string {
    return 'Hello World!';
  }
}