it's probably because you owe money for the time you've been on supply with them. (you are a deemed customer as pp supplied the property you moved into)
Have you registered and tried to pay the bill, or haven't you had a bill yet?
Here is some info on what to do
Moving home - Is your new home on Pure Planet’s energy supply?
If you’ve recently moved into your home (congratulations!) and it's on Pure Planet’s energy supply, we can get you up and running in no time.
Moving house is a tricky time, so we want to help as much as we can.
Once you’re in your new home, you’ll receive a letter in the post telling you about us.
All you have to do is go to the Pure Planet website or download our app via the App Store or Google Play.
You'll see a button called ‘Moved into a Pure Planet home?’ which will guide you through joining up.
The process will take less than a minute.
You’ll then be contacted by us via email. Then, you’re all done. You’re on cheaper, renewable energy.
Any other questions? Ask the Community