PLATFORM_MAC_ENABLE_EXPERIMENTAL_NANITE_SUPPORT=1
in UEBuildMac.cs (Engine/Source/Programs/UnrealBuildTool/Platform/Mac/UEBuildMac.cs). UE_EXPERIMENTAL_MAC_NANITE_SUPPORT
in spirv_msl.hpp
.UEBuildMac.cs
.bSupportsNanite=true
and bSupportsUInt64ImageAtomics=true
in Data Driven PlatformInfo.ini (Engine/Config/Mac/DataDrivenPlatformInfo.ini).