Page Notes comes with three:

• Alt + P — open Page Notes on the page you are looking at
• Alt + O — open All Notes
• Alt + S — toggle the side panel

If one of them does nothing, your browser has already given that combination to something else. A shortcut an extension asks for is a suggestion, not a reservation: if the combination is already taken — by the browser itself, or by another extension you installed earlier — the command simply arrives unassigned. Nothing is broken, and the extension has no way to tell that it happened.

You can see which ones are assigned, and change any of them:

• Chrome and other Chromium browsers — go to chrome://extensions/shortcuts (edge://extensions/shortcuts on Edge), find Page Notes, click the box beside the command you want and press your combination.
• Firefox — open about:addons, click the gear icon at the top right, and choose Manage Extension Shortcuts.

The page shows you when a combination is already in use, so you will know before you commit to one.

You can also set a shortcut for a command that has none, or clear one you would rather not have. Page Notes works the same either way; the shortcuts only save you a click on the toolbar icon.