Did you try reading the error message? It's telling you exactly what's wrong, and it's a straightforward message.
It's pointing at your "while" condition and telling you that you can't use void there. You understand why, right? If you have while(X < Y), then X and Y must both be numerical values. You know that, right? So, it's pointing at that and complaining about something being void. You should be able to figure out what is void there.
And now that you know that, it should be obvious why it's an error.