If anyone from Apple is reading this, please, please, please restore compatibility with Mojave before Xcode 11.4 is finished! I don't want to have to downgrade from Mojave just to continue using Xcode 11.

Thanks, but that just accesses the velocity. My question is, how do I change the velocity?

I had the same problem, and found a workaround. This appeared in the build log: note: did not find header 'XCTest.h' in framework 'XCTest' (loaded from '/Applications/Xcode-beta.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator14.0.sdk/Developer/Library/Frameworks') Sure enough, the XCTest.framework in that path was missing a Headers directory. Either they forgot to copy it in, or they're linking against the incorrect framework. I was able to fix the problem by adding a symbolic link to the current Xcode's XCTest's Headers directory: cd /Applications/Xcode-beta.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator14.0.sdk/Developer/Library/Frameworks/XCTest.framework ln -s /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/Library/Frameworks/XCTest.framework/Headers (If your Xcode installations aren't in /Applications, don't forget to change the top of those paths appropriately for your installation.) Once I did that, then my project built correctly.

I do, and yes, those flags will not work. For visionOS, the flags changed to -target arm64-apple-xros1.0 for the device, and -target arm64-apple-xros1.0-simulator for the simulator.

That warning occurs when the library was built to require a newer macOS version than the application requires.