Sims 4: Cannot Save Sims in CAS

@jordyndda  It sounds like you already did almost everything that would be recommended.  But it might be worth running the Origin reset tool and disabling Origin in-game, if you haven't already.  And it's a good idea delete or move your TS4 game folder before each troubleshooting step, just to keep things simple.

https://help.ea.com/en-us/help/origin/origin/how-to-reset-origin-on-mac/

Someone with more expertise than I have will probably have more ideas when she shows up; in the meantime, it would be helpful to know the specs of your mac, as described in the following post.  Please also list the packs you have installed.

https://answers.ea.com/t5/The-Sims-Mac/READ-FIRST-Sims-3-Mac-Help-Guides-and-Troubleshooting/m-p/4756624#M4

I can try resetting Origin. I reset it a few days before this issue started in my game so I'm not sure if that will work. 

Specs of my Mac: 

• Operating System: macOS High Sierra Version10.13.6
• Mac model: MacBook Pro 13inch mid-2012
• Processor: 2.5 GHz Intel Core i5
• Memory (RAM): 4 GB 1600 MHz DDR3
• Graphics: Intel HD Graphics 4000 1536 MB

Packs Owned: 

• Get Together
• Get to Work
• Seasons
• Cats and Dogs
• Vampires
• City Living
• Outdoor Retreat
• Spa Day
• Toddler Stuff
• Laundry Day
• Parenthood
• Jungle Adventure
• Dine Out
• Fitness Stuff
• Kids Room Stuff
• Romantic Garden Stuff
• Spooky Stuff
• Cool Kitchen Stuff
• Perfect Patio Stuff
• Luxury Party Stuff

@jordyndda  Do you really only have 4 GB RAM installed?  From your other specs, I'd expect it to have shipped with 8 GB.  That may be exactly the problem, though.  TS4 is not my area of expertise, but from your specs, I would suspect that your computer is running out of memory.  You have only 4 GB RAM installed, and High Sierra could be taking almost 3 GB of that.  TS4 is easier to run on weaker hardware than its immediate predecessor, but a little over 1 GB of available memory isn't nearly enough for the game to maneuver, especially in CAS.

An interesting thought. My computer is one of the older models that did ship with 4GB RAM. I might have to look into this a bit more. It's run for quite some time without an issue with High Sierra installed, but maybe it has come to that point. 

@jordyndda  You can keep an eye on RAM use in the Activity Monitor (in Applications\Utilities).  You'd have to be in windowed mode to switch back and forth, of course, but you could see how close you were to running out of memory at any given time.  This guide is for TS3, which requires frequent RAM purges during gameplay (at least in mac OS), but it might help you too:

https://bluebellflora.com/resetting-ram-during-gameplay/