It seems that it is coming from the new field in the target setting - "package name" that was not there in my previous xcode version. It is searching this name as main executable file inside the .app package. This happen only when compiling for a simulator. example: your package was (legacy) "booboo", so it is searching now for booboo.app/booboo, which was not auto generated by the new compile system... someone want to elaborate on that some more?...

its unbelievable to me, every few years that i come back to code with IOS, how so many people get stuck with so many issues - just to do simple tasks like compiling code that was previously compiling... or searching where the new settings that Apple want us to tick. and still - fighting and spending hours resolving issues with every xcode new version and ios new updates. so many wasted hours that we could all build other amazing things...