I remember a fellow player having this issue and what we found is that their Saves, library and exports folders had hit the limit of what the game could hold.
When I have issues with my game and need or want to troubleshoot I start by saving a copy of my Sims 3 folder to my desktop. That way I have a backup of all my Sims 3 files if I need them. The folder can then be deleted after problem has been solved.
Go to: C:\Users\Username\Documents\Electronic Arts - right-click the Sims 3 folder > Copy/Paste to desktop, usb drive or external hard drive.
With trying to troubleshoot saving issues. I remove most of my save files from my Saves folder.
C:\Users\Username\Documents\Electronic Arts\The Sims 3\Saves
Since you're having issues saving lots to the library. Remove the files from both the Library and Exports folders.
Remove all the library files in your library folder.
C:\Users\Username\Documents\Electronic Arts\The Sims 3\Library
Remove all the files in your Exports folder.
C:\Users\Username\Documents\Electronic Arts\The Sims 3\Exports
Deleting the files within these folders will hopefully allow you to be able to load and save a renovated venue/lot.
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Here's another tip that I sometimes = most often lol use, in case the other one doesn't work.
Create a New Sims 3 folder. This serves the purpose of refreshing the Sims 3 game and launcher and it's cache files in the DCCache folder.
To create a new Sims 3 folder, go to: \Documents\Electronic Arts - Right-click on the Sims 3 folder and select Rename. I usually rename it something like Sims3-old or Sims3-1. Everything is safe in your old Sims 3 folder.
Launch the game and exit it so it creates a new folder.
Start a new test save in Sunset Valley for example to see if it still has issues.
– Testing Lots: via Edit Town mode, look at the in-game library - Empty lots, Community lots.
– Testing Build/Buy: via Edit Town mode pick a lot and go into Build/Buy.
– Testing CAS: Load CAS and check for installed items.
Do your lots, build/buy items, and/or CAS items show up?
Quit without saving.
If all is good, it might be something in your old folder got corrupted. You can then start copying files/folders over from your old folder into your new one. –– Be sure to check your mods and custom content to make sure they aren't causing conflict with the game.
If you need help in knowing what to add, see the post linked below:
Sims 3 - How To: Saving Folders/Files
https://www.carls-sims-4-guide.com/forum/index.php?topic=16414.msg313134#msg313134