Hi! I understand that when you try to highlight text in the Libby app on your Mac running OS X 10.15.7 and select a color, the highlight disappears. Let's see if we can get this sorted out together.

To start, can you tell me a bit more? For example:
- Are you using the Libby app downloaded on your Mac or accessing Libby through a web browser?
- Does this happen with every book or only specific titles?
- Have you tried restarting the app or your Mac to see if that helps?

Meanwhile, here are some general steps we can try:

1. Restart the App and Device: Close the Libby app completely. If you're using a browser, close that tab or browser window. Then restart your Mac and try highlighting again.

2. Check for Updates: Ensure your Libby app or your browser is up to date. Sometimes older versions have bugs affecting features like highlights.

3. Remove and Re-add Library Cards: Sometimes the connection to your library cards can cause syncing issues. In Libby, go to the Menu, then Manage Cards, remove your cards, and add them back.

4. Try Highlighting Another Title: Test if highlighting works on a different book. This can help determine if the problem is specific to one book.

5. Clear Browser Cache (if applicable): If using Libby on a browser, try clearing the cache or using a different browser to see if that resolves the issue.

If none of these work, it might be a syncing or account issue, or possibly a bug with the version of Libby you're using.

Could you let me know what you’ve tried so far or any more details you’ve noticed? That way I can customize the help better for your setup.