Why am I getting the message "WDDocument error 1" on my Mac when I try to open a new document?

You can have only one Vectornator App installed.

So all you need to do is to delete the old version to solve the problem (see image).
But first, save your unsaved documents in some folder and then delete the App. If the documents are saved in the Finder, they will stay there, even if the app is uninstalled.