Create a completion block that returns nothing but executes another function when completed?

I'm trying to create a completion block that can execute another function after its completed, in this case it's a tableview reload. I get the error :

'async' produces '()', not the expected contextual result type 'Bool'

This is the function:

  func appendAllData (completion: () -> Bool) {

    if self.movieDetailsData?.poster != nil {
      if let posterImage = self.movieDetailsData?.poster {
        self.posterArray.append(posterImage)
      }
    }
    if self.movieDetailsData?.overview != nil {
      if let overview = self.movieDetailsData?.overview {
        self.overviewArray.append(overview)
      }
    }

    if self.movieDetailsData?.releaseData != nil {
      if let releaseDate = self.movieDetailsData?.releaseData {
        self.releaseInfoArray.append(releaseDate)
      }
    }

    if self.movieDetailsData?.runtime != nil {
      if let runtime = self.movieDetailsData?.runtime {
        self.releaseInfoArray.append(String(describing: runtime))
      }
    }

    if self.movieDetailsData?.genre != nil {
      if let genre = self.movieDetailsData?.genre {
        if genre.isEmpty {
        } else {
          self.releaseInfoArray.append(genre[0].name)
        }
      }
    }

    if self.movieDetailsData?.budget != nil {
      if let budget = self.movieDetailsData?.budget {
        self.boxOfficeArray.append(budget)
      }
    }

    if self.movieDetailsData?.revenue != nil {
      if let revenue = self.movieDetailsData?.revenue {
        self.boxOfficeArray.append(revenue)
      }
    }

    if self.movieDetailsData?.homepage != nil {
      if let homepage = self.movieDetailsData?.homepage {
        self.homePageArray.append(homepage)
      }
    }

    if self.movieDetailsData?.images != nil {
      if let images = self.movieDetailsData?.images {

        let posters = images.backdropImages
        for poster in posters {

          self.imageArray.append(poster.filePath)
        }
      }
    }
  }

This is how it's used:

 self.appendAllData(completion: { _ in

  DispatchQueue.main.async { //error here: 'async' produces '()', not the expected contextual result type 'Bool'
    self.detailTableView.reloadData()
  }
})