This appears to be a bug in Windows 10 for Windows Phone 8 apps.

Confirmed fix from some customers is:

  1. Sign out of the Store
  2. Restart your phone
  3. Then find the app in the Store
  4. Select Buy
  5. Sign in when requested
  6. The app should now be activated correctly now.
  7. If this doesn't work then please try again.
  8. Failing that please contact us.