What's wrong with my code?

The questions of this code challenge is as follows:

We learned about instance methods before. For this task, add an instance method to Post.

Name the method description. It takes no parameters and returns a string that describes the post instance. For example given a title: "iOS Development", author: "Apple", and a tag named "swift", the description would read as follows"

"iOSDevelopment by Apple. Filed under swift"

Once you have an instance method, call it on the firstPost instance and assign the result to a constant named postDescription

I have tried coding it directly on Xcode already and it works. However, it still reply me with an error. Could anyone help me figuring it out? Thank you

struct Tag {
    let name: String
}

struct Post {
  let title: String
  let author: String
  let tag: Tag

  init(title: String, author: String, tagName: String) {
    self.title = title
    self.author = author
    self.tag = Tag(name: tagName)
  }

  func description() -> String {
    return "\(title) by \(author). Files under \(tag.name)"
  }
}

let firstPost = Post(title: "iOS Development", author: "Apple", tagName: "Swift")

let postDescription = firstPost.description()