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 vSAN or virtual flash resource, a local flash disk does not appear on the list of disks to be used.
This problem might occur when flash disk intended for use with either feature has been already formatted with VMFS. Virtual flash and vSAN cannot share the flash disk with VMFS or any other file system.
Also, because virtual flash and vSAN are mutually exclusive consumers of flash disks, both features cannot share a flash disk. If the flash disk is already claimed by one feature, for example vSAN, you cannot use it for another, such as virtual flash, unless you release the disk.
Use only unformatted flash disks for virtual flash resource and vSAN 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 vSAN. If the disk is claimed by vSAN, remove the disk from vSAN. The flash disk is released from vSAN and becomes available on the list of disks to use with virtual flash. For information about removing disks from vSAN, see the Administering VMware vSAN documentation.
If you intend to use the flash disk with vSAN, 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 vSAN. 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.