Hi,
Your first issue is that you are likely dragging the gameobject itself instead of the component. Drag the component toolbar and then it will know that you want to get or set that specific component.
As for the coins, Each coin shoudl be assigned a unique ID and when the user check that coin, save that this coin is taken so that when the scene reloads, each coins check whether they should disappeary. OR you have a manager that enables coins that are not yet taken. In that casem you would edit your level with some placeholders whee coins should be instanciated, and these placeholders would have a unique Id.
Bye,
Jean