Hello! I understand you're having trouble downloading the Jacquie Lawson Advent Calendar app on your Macintosh with OS X 10.15. Let's get to the bottom of this together.

First, could you tell me a bit more? Are you trying to download the app from the Jacquie Lawson website directly, or from another source like the Mac App Store? Also, do you see any error messages when the download fails? That will help me pinpoint the problem.

In the meantime, here are some common steps we can try:

1. Check Your Internet Connection
Make sure your Mac has a stable internet connection. Try loading other websites to confirm.

2. Verify Available Disk Space
Sometimes downloads fail if there isn't enough free space. On your Mac, click the Apple menu > About This Mac > Storage to see if you have enough free space for the app.

3. Browser Cache and Cookies
If you’re downloading directly from the website via a browser, clearing the browser cache can help. Since you’re on OS X 10.15, commonly Safari is used. To clear Safari cache: Safari menu > Preferences > Privacy > Manage Website Data > Remove All.

4. Administrator Rights
Although the detailed FAQ for Windows notes administrator access is required, it’s good to ensure your user account on your Mac has sufficient rights to install applications.

5. Try Another Browser
If the download repeatedly fails in Safari, try downloading via another browser like Chrome or Firefox, just to rule out browser-related issues.

6. Security Settings
Sometimes Mac security settings block installation from unidentified developers. After downloading, if you see a message that the app can't be opened, try going to System Preferences > Security & Privacy > General tab, then click "Open Anyway" if the app is blocked.

If these steps don’t resolve the problem, please share any error messages or describe exactly what happens when you try to download, so I can assist further.

Are you comfortable trying these steps now? Let me know how it goes!