Why can't I use the front camera?

Not all devices support 60fps shooting with the front camera. If you have a device older than the iPhone 6, you will not be able to use the front camera for 60fps shooting.

The app keeps re-focusing

You are most probably using an iPhone 5 or 5S, or shooting in low-light conditions.

Since the app is shooting at a high framerate, shooting in non-optimal conditions can throw off the auto-focus. You can tap anywhere on the screen to lock focus momentarily, or you can switch to manual-focus for full focus control.