When you install ESXi, the installer will attempt to claim ~4GB of local disk space to use for the "scratch" partition (gets mounted under /scratch). If the installer is unable to find ~4GB of free space, then it will place scratch on RAMDISK (in which case /scratch gets linked to /var/tmp). Scratch is used primarily for storing log files, but also has other uses.
The warning is telling you that in your case "scratch" is in RAMDISK and advising you that you should change this to a persistent storage location. The host will run fine with scratch in memory, however, each time you reboot you will lose your logs. Hence the recommendation to move scratch to a persistent location.
I can think of a few reasons why the installer may fail to create a "scratch" partition:
1. The installer will not create scratch on a USB/SD (as these devices are write sensitive, the installer avoids using them for scratch).
2. The host was an ESX host that has been upgraded to ESXi.
3. There is no local storage (i.e. stateless ESXi (auto deploy).
4. The installer fails to recognize a local disk device as local.
Regardless the cause, understand the installation is fine and the host will run w/out issue. But you should take steps to relocate the scratch to a persistent datastore. For more info please refer to:
Booting ESXi off USB/SD | VMware vSphere Blog - VMware Blogs
Regards,
-Kyle