when i used PC platform to package my iOS project,i had this problem,and i added two line,but it did’t work.
The problem is when you use PC platform to package ios projects, it will automatic generation a shell called SignProject.sh in mac.
xcodebuild command line is on the last line of the file.
so you need to change your source engine file(if your engine is complied by you ,not epic provided).You need to add " -UseModernBuildSystem=YES" in IOSToolChain.cs and recompile your engine,just change the .cs file is not working because it may already compiled into a DLL.
