Let’s analyze what this error means:

Cannot read property ‘0’ of undefined

This means we’re doing something[0] somewhere, and something is undefined. You’ve already identified the line where this is happening:

return <BoardSquare value={items[e]} onClick={() => onClick(e)} />

Here the something is items. So where is items coming from? Looks like the props of the component:

function Board({ onClick, items }) {
  // ...
}

So what passes in the items prop? Your MainGame component:

<GameBoard onClick={() => handleClick} />

Ah ha! It’s not passing an items prop. That means it will be undefined.