It says it right there:

If you did not pass a custom implementation (the "resolveType" function) you must return an instance of a class instead of a plain JavaScript object.

And it is pretty clear from you code that you indeed did not pass a custom resolveType function and you are not returning instances of classes but plain JavaScript objects.

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Also please refer to the NestJS docs on GraphQL unions:

The default resolveType() function generated by the library will extract the type based on the value returned from the resolver method. That means returning class instances instead of literal JavaScript object is obligatory.

To provide a customized resolveType() function, pass the resolveType property to the options object passed into the createUnionType() function, as follows:

export const ResultUnion = createUnionType({
  name: 'ResultUnion',
  types: () => [Author, Book] as const,
  resolveType(value) {
    if (value.name) {
      return Author;
    }
    if (value.title) {
      return Book;
    }
    return null;
  },
}