A local flash device becomes unavailable for virtual flash resource or Virtual SAN configuration when it is formatted with VMFS or any other file system.
When you attempt to configure either Virtual SAN or virtual flash resource, a local flash disk does not appear on the list of disks to be used.
This problem might occur when a local flash intended for use with either feature has been already formatted with VMFS. Neither Virtual SAN nor virtual flash can share the flash disk with VMFS or any other file system.
Also, because virtual flash and Virtual SAN are mutually exclusive consumers of flash disks, both features cannot share the same flash disk. If the flash disk is already claimed by one feature, for example Virtual SAN, you are not be able to use it for another, such as virtual flash, unless you release the disk.
Use only unformatted flash disks for virtual flash resource and Virtual SAN configuration.
Avoid formatting the flash disks with VMFS during ESXi installation or Auto Deploy.
If the flash disk is already formatted with VMFS, remove the VMFS datastore. For information see the vSphere Storage documentation.
To use the flash disk as a virtual flash resource, do not claim this disk for Virtual SAN. If the disk is claimed by Virtual SAN, remove the disk from Virtual SAN. The flash disk is released from Virtual SAN and becomes available on the list of disks to use with virtual flash. For information about removing disks from Virtual SAN, see the Administering VMware Virtual SAN documentation.
If you intend to use the flash disk with Virtual SAN, do not use the disk for a virtual flash resource. If the flash disk is used as the virtual flash resource, remove the virtual flash configuration. The disk becomes available for Virtual SAN. See the vSphere Storage documentation.
Another reason that makes flash disk unavailable is when ESXi cannot detect the disk. See Local Flash Disks Are Undetectable.