The current Mac version of GoodNotes is removed from sale temporarily because it is not compatible with GoodNotes 5.
We're working on a new version for GoodNotes for Mac, that will be compatible with GoodNotes 5. You can read more about it here. If you urgently need a Mac version of GoodNotes 5, sign up for early access. We're inviting new batches of sign-ups usually once a week.