Re: [MOD RELATED] Icons are blank squares/ UI Glitching
@emmalouisehill That sounds like one of the side effects of an outdated version of the UI Cheats mod. If you have it, please remove it, delete localthumbcache.package as well (also in your Sims 4 folder in Documents\Electronic Arts), and see whether the game works normally. If it doesn't, remove your entire Mods folder, delete localthumbcache again, and test. You don't need to save your progress, so don't worry about any missing objects warning you see.
If this doesn't help either, please move the entire Sims 4 folder out of Documents\Electronic Arts and onto your desktop. When you launch the game, a clean folder will spawn with no content. Don't add anything to this folder yet; just start a new save and see whether Build/Buy works normally.