What version of macOS are you using ?
You don't need to login to get the Open Source version of Qt.
The /Application folder is protected because it's shared among all computer users. You should rather keep the default installation path in your user's folder.
There's no need to select everything especially if you intend to only develop on macOS. Just select the macOS build.
What exactly did the Kit page show once you had that message in Qt Creator ?
By the way, it's Qt, QT stands for Apple's QuickTime which is likely not what you want to develop with.