[C#] Simple code error storing data to a List

I am pretty new to C# so many things I have never done before, this is one.

I have the following code were I am testing to save data to playerPrefs using BinaryFormatter. However, the problem i have is how to store the "CardData" to the cardSavedData -list.

I do get the following message:

on this line below: "cardSavedData.Add (CardData);"

I am...

[C#] Simple code error storing data to a List