You need to correct your code to wait for the list to be downloaded:
List<Item> list = await GetListAsync();
Also, make sure that the method, where this code is located, has async modifier.
The reason why you get this error is that GetListAsync method returns a Task<T> which is not a completed result. As your list is downloaded asynchronously (because of Task.Run()) you need to "extract" the value from the task using the await keyword.
If you remove Task.Run(), you list will be downloaded synchronously and you don't need to use Task, async or await.
One more suggestion: you don't need to await in GetListAsync method if the only thing you do is just delegating the operation to a different thread, so you can shorten your code to the following:
private Task<List<Item>> GetListAsync(){
return Task.Run(() => manager.GetList());
}