If a yellow notepad appears while attempting to use Astropad, this typically means that Apple Ink is enabled on your Mac. Here are the Apple support articles for changing your Ink preferences:
You can also access Ink with the following steps:
A pencil icon will appear in the right part of your menu bar to indicate that Ink is enabled. It will now also be available in System Preferences. You can open Ink Preferences from the icon or directly within System Preferences to enable and disable.