I don't have a Mac, so not sure of a solution.
But it sounds to me that you have another program running that is 'high-jacking' the spacebar shortcut.
I have no idea what it might be, but can you stop all other applications (especially those running in the background) and have just Archicad running.
One of the forum moderators.
Versions 6.5 to 25
Dell XPS- i7-6700 @ 3.4Ghz, 16GB ram, GeForce GTX 960 (2GB), Windows 10
Dell Precision 3510 - i7 6820HQ @ 2.70GHz, 16GB RAM, AMD FirePro W5130M, Windows 10