Okay, so I've noticed a couple of problems.
First, on line 45, toward the end of
componentDidUpdate() in the
App class, you call the
minimax function with the board state, and
this.state.symbol, which is undefined. You'll need to find a way to get the CPU's symbol to pass to the function to get it to work correctly.
Second, once I fixed that, too many moves are being made. I'm too tired to pinpoint exactly what's causing it, but I think it's due to the fact that
componentDidUpdate() is being fired too many times, so you may want to move the cpu player logic out of that function and into something that you can make sure is only called once.