How to fix empty schema.gql file?

I want to connect my frontend with backend by apollo server.

Schema is generated in backend main folder and i have following settings:

app.module:

GraphQLModule.forRoot<ApolloDriverConfig>({
      driver: ApolloDriver,
      autoSchemaFile: "schema.gql",
      sortSchema: true,
      playground: false,
    }),

main.ts:

import { NestFactory } from '@nestjs/core';
import { GraphQLSchemaHost } from '@nestjs/graphql';
import { AppModule } from './app.module';
import { ValidationPipe } from '@nestjs/common';
import { ApolloServer } from '@apollo/server';
import { startStandaloneServer } from '@apollo/server/standalone';

async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  app.useGlobalPipes(new  ValidationPipe());

  
  const server = new ApolloServer(
    { schema: await app.get(GraphQLSchemaHost) }
  );

  await startStandaloneServer(server, {
    listen: { port: 3000 },
  });

}
bootstrap();

error from terminal:

Error: Expected {} to be a GraphQL schema.

How can i fix it?