Cannot find "DatabaseError" in Scope

public func getAllConversations(for id: String,completion: @escaping(Result<[Conversation   ],Error>) -> Void) {
    database.child("user").child(id).observe(.value, with: { snapshot in
        guard let value = snapshot.value as? [[String:Any]] else {
            completion(.failure(DatabaseError.failedToFetch))
            return
        }
    })
}

I'm getting error:

cannot find DatabaseError in scope.