where is this option existed in 2020
"temp" files is not related to RAM memory. You should increase the Virtual Memory (the swap file) instead. And clean up often. If you have enough free disk space, start with 20 Gb and set max to 30 or 50 Gb. Surely you don't need more.
For clean up temp files, go to Windows Start menu, type %temp% without any program open or in use, and on the following list, select all and delete it (it's better to do it after restart the computer). It's enough to do it once on a week.
of course it's supposed that you're using 64 bits versions of Windows and CorelDRAW
I'm not a computer whiz or anything, but I think the reason why you can't allocate memory anymore is because Corel is now a 64 bit program so it automatically chooses it's own RAM. 32 bit and lower had to have the RAM levels set. I know this because of Minecraft, so...
Although it was already explained previously, remember that the answer is to increase the virtual memory of Windows, which has no relation to RAM
utterly no use. i altered no use no use no use. just not say that we are not facing such issues. issues arises those who works on large files.