Hi there! I want to make sure I understand your situation clearly—when you say the Substack app "does nothing," could you tell me a bit more? For example, does the app fail to open at all, get stuck on a loading screen, or open but not show any content? Also, are you logged into your account, or is that where things get stuck?

Knowing these details will help pinpoint the issue and guide you better. Meanwhile, here’s a general approach we can take to troubleshoot on your Mac running OS X 10.15.7:

1. Confirm App Compatibility: The Substack app is primarily designed for iOS devices (iPhone/iPad). Did you download it via the App Store on your Mac, or are you using an iOS emulator? If it's not a native macOS app, that might explain why nothing happens.

2. Basic Restart: Sometimes a simple restart of your Mac can clear out system hiccups.

3. Check for Updates: Ensure your macOS and any app you're using are fully updated. But since you're on Catalina (10.15.7), macOS is a bit older, which might limit some app compatibility.

4. Try the Web Version: Since Substack is primarily a web-based platform, try accessing your Substack account via your browser at https://substack.com to check if your subscriptions load and function normally there.

5. Reinstall (if possible): If you installed an app from the Mac App Store, try deleting it and reinstalling to clear cached problems.

6. Network Check: Make sure your network connection is stable and no VPN or firewall is blocking the app's connection.

Given that the Substack app is mainly for iOS, if you want the full experience on a Mac, the web version is the best choice.

Could you share how you installed the app and what exactly happens when you open it? That way, I can provide more targeted help. Are you open to trying these steps now?