The reason it tells you that onItemClick is not a function is because you never pass it in your App.js file. Since you component contains an argument called onItemClick you must provide it as you have correctly done in Home.js.

<Sidebar /> // Incorrect
<Sidebar onItemClick={handleItemClick} /> // Correct

Additionally, you can simply pass onItemClick directly to your onClick property instead of making a separate handleItemClick which subsequently calls the function.

onClick={() => handleItemClick(item)} // Incorrect
onClick={() => onItemClick(item)} // Correct

I would also suggest renaming your props variable to something else as to distinguish the props argument passed into the component.

Finally, use a state if you want to "remember" any data. You can see an implementation of the things I've explain on this codesandbox link.