Hi there! I understand you're experiencing crashes with Substack after about 20 seconds. Let's work together to get this sorted out.

Can you let me know if you're using the Substack app or accessing it through a web browser on your Mac? Also, have you noticed any specific actions that lead to the crash, like posting or editing content? This information will help me guide you better.

On the app. No posting by me

Thank you for clarifying that you are using the Substack app and not posting anything. Let's try a few steps to see if we can resolve the crashing issue you're experiencing.

### Step 1: Restart the App
Sometimes, a simple restart can help. Close the Substack app completely by swiping it up from the app switcher. Then, reopen the app and see if the issue persists.

### Step 2: Update the App
Check if there’s an available update for the Substack app. To do this:
1. Open the App Store on your Mac.
2. Click on the Updates tab in the sidebar.
3. If you see Substack listed, go ahead and click Update.

### Step 3: Clear Cache (if applicable)
Though there may not be a straightforward way to clear cache in all apps, you could try deleting the app and reinstalling it. This often helps in removing any corrupted data. To do this:
1. Open Finder and go to the Applications folder.
2. Locate the Substack app and drag it to the Trash.
3. Go to the App Store and download the Substack app again.

### Step 4: Check for System Updates
Make sure your OS is up to date as well:
1. Click on the Apple menu in the top left corner.
2. Select System Preferences, then click on Software Update.
3. If an update is available, install it.

### Step 5: Consider Compatibility
If the problem continues, it might be related to compatibility issues with the version of the Substack app and your OS X 10.15.7. Substack occasionally updates their app for newer versions, so double-check their substack website for any compatibility notes.

Could you give these steps a try and let me know the outcome?