Out of memory condition prevents the SSD from being assigned to VirtuCache

If you are not able to attach an SSD to VirtuCache, and if there are no other obvious issues listed here that prevent the SSD from being assigned to VirtuCache, then its most likely that there is not enough memory available to accomplish this task. VirtuCache runs into this out-of-memory condition especially (but not necessarily) if the SSD is larger ( > 3TB) and there are VMs running on the host.
Moving the host to maintenance mode and then assigning the SSD to VirtuCache will fix this issue.

If you want to confirm that out-of-memory condition is indeed the reason why the SSD is not being attached to VirtuCache, then check the vmkernel logs for the following signature:
“Unable to register the cache – ca”
You can either do this by downloading the support bundle and searching within vmkernel.log or you can run this command on the ESXi host CLI: dmesg | grep -i “Unable to register the cache”

VirtuCache requires larger amounts of memory (that in turn depends on the SSD size) only during this initial step of attaching the SSD, and not during regular VirtuCache caching operations after the SSD has been successfully assigned to VirtuCache.