UIKit.h not found error

I have had the same issue with being unable to compile project due to UIKit.h not being found + "unable to load standard library for target..."

The solution for me was very simple - to quit (CMD+Q) and relaunch Xcode. I have found out that with Xcode opened, macOS seems to have removed command line tools for Xcode because it was preparing for a update and relaunching Xcode triggered the "install additional components" update and it made the problem disappear.

Like Faul Textor said, most likely your XCode updated to 12.1 but your Command Line Tools didn't. You need to quit XCode completely and then start it again and you should get a pop-up saying "Install Additional Components" Sorry, this should have been a comment to his reply but I cannot do that.

It sounds like you removed the UIKit.framework from your project at some point. Check if it is listed in your project navigator. If it's not there, go to your project in the project navigator pane, select your target, go to the build phases tab, click on link binary with libraries, hit the plus button, find the UIKit.framework there and add it to your project.

EDIT: This answer suggests that there mayb be an issue with your framework search paths build setting. Check that out and if the path is empty and it still doesn't work, a re-install of Xcode would fix it, I think.