I just run into an issue today after upgrading to Xcode 26. Could be different with UE 5.6 but this worked with 5.5:
Edit the file /Users/Shared/Epic Games/UE_5.5/Engine/Config/Apple/Apple_SDK.json and change the line for MaxVersion to “MaxVersion”: “26.0.0”. In case you later get an error related to Xcode Metal compiler go to Xcode → Settings → Components, look for Metal toolchain and click download.