Or that you have lots of free disk space, but the currently running apps need such a large amount of virtual memory address space that macOS fails to provide. So, it takes up more and more memory until your hard drive is full and there is no room for the swap files, the system will report that it has run out of application memory. When running an app on Mac, if all physical RAM are used, macOS will create swap files, which takes up disk space. Reason 2: Your boot disk has little free space for running apps If that’s the cause, to quit or force quit the program may simply solve it. It’s a common practice to run several apps on your Mac at the same time however, accumulation of excessive apps on the background will take up more and more space and finally leads to the problem of “system has run out of application memory”. Reason 1: Too many apps run simultaneously There are four possible reasons account for the problem:

When your Mac says system has run out of application memory, it means that your computer doesn't have enough memory required by applications to run. What Does System Has Run Out of Application Memory Mean?

Mail on imac keeps freezing