Difficulty setting up for macOS 64 bit compiling

Could somebody give me some advice please? I am having difficulty setting up a connection profile for MacOS 64-bit. I am running Delphi Rio and am compiling to a 2011 Macbook running High Sierra. When I try to compile my 64 bit profile the Add New SDK dialog pops up with macOS-64bit selected. However when I click on OK it does not compile and I get a compilation error "Failed". Interestingly if I edit my MacOS 32 bit connection the Connection Profile Manager has the platform as macOS-64bit (Other platforms available are Windows - no MacOS 32bit listed). This connection works and compiles to my mac but the result is still a 32 bit build. Maybe I cannot compile 64-bit to a Mac of this era? Any help greatly appreciated!