Issue:
Cannot download Expansion, Stuff and Game Packs for Sims 4 (November 2020). When I click the orange Download button nothing happens.
Cause:
You’re running the beta/technical preview of Origin. You may not even realise this if you have just downloaded Origin but if you look in the top right corner of the Origin window you will see Technical Preview.
Fix:
Go into Application settings in Origin and disable the technical preview then restart Origin. 
Edit July 2023 – this issue is now fixed. See this post for the update.
If you’re trying to install your game to an external drive you may have issues if you’re running macOS 10.15 Catalina or later. Here are quick steps on how to successfully install your game to an external drive:
1. In System Preferences > Security & Privacy > Privacy click on Full Disk Access and add Origin if necessary.
2. Whilst in the Privacy tab make sure Origin is showing in Files and Folders
3. Open Origin and go to Preferences > Installs & Saves > Game Library Location and click on Change Folder
4. On the left menu click on Computer then double click on the ‘/‘ that appears next to a picture of your hard drive. You should see four folders – Applications, Library, System, Users
5. In the field at the bottom, where it says Directory, type in /Volumes/[the name of your external] then hit Enter/Return or click Choose.For example, my external is called Backup so the file path is /Volumes/Backup 
6. The window will disappear and reappear again but you may not see your external. That’s ok, quit Origin then reopen it. You should get a Gatekeeper message popping up asking for permission to access files on a removable volume. Just click Ok
7. Now repeat steps 3 to 5 and you should see the contents of your external in the window
The Sims 4 received a major update for Mac users in April. Not in terms of game content but changes to the way it is installed to comply with Apple’s notarisation terms for Catalina, enhancing the security of your system. As a by-product of the notarisation you may now see a greyed out Finalizing option under the game in your Origin games library when you install content, install a patch, or repair the game:
This is normal and is the OS checking that the download in Origin is all safe and ok to run. It can show as finalizing for any length of time, depending on your Mac hardware. The more powerful your CPU, the quicker it will complete the checks. You may notice the fans ramping up in your Mac during this process, that is normal as it is using a significant amount of CPU so is in need of sufficient cooling during the process. Occasionally the button doesn’t change and seems stuck on finalizing. Don’t panic if this is the case! Try the following to fix it:
This will automatically launch Origin and then the game. Remember you have two Applications folders, the system one and your user account one. By default Origin currently installs the game to your user Applications folder unless you have changed the setting. The locations can be found here:
Bluebellflora 