I use MacOS, too, without any issues. Maybe your system rights for MacOS don't allow the software to change the settings. I don't know this problem, but I heard that it can be possible. You can check right-click openLCA.app "show package contents", click on "contents", click on "eclipse" and open/edit the openLCA.ini. You will find their the allocated RAM storage, for example "-Xmx32768M".