In fact, Xcode 12.4 does not support iOS 14.5.1, but if you find on the web the 14.5 zip folder which contains this version, you can upload your app to the device, the thing is that you are not able to debug the application. Do you think apple will release an 12.4 version that supports 14.5.x ? https://github.com/filsv/iPhoneOSDeviceSupport

I have the same problem, I was able to upload the app finding on. the web the 14.5 zipped file, but I am not able to debug. The connection fails when it tries it. Have you solved the problem ? I am downloading Xcode 12.5, then, I will extract the 14.5.1 directory if it exist.