I hope this is working for you now but if not it could be this. When I first started using PHD I got this error as well until one day I figured out what I was doing wrong. I was trying to use PHD while I had another program connected to the guide camera running in the background. So I had a situation where the camera would be trying to guide and capture simultaneously. Now to the experienced ones this may be a no brainer but when you're new to all of this and learning it for the first time you'd be surprised at some of the silly things you do. Like trying to take subs with the lens cap still on (A friend of a friend of a friend did that one, not me I swear).
So maybe check that you're not running the guide camera in another program at the same time.