An administrator is tasked with replacing the existing Witness Host for a 2-node vSAN Edge...

A witness host for a 2-node vSAN cluster must be external to the 2-node vSAN-enabled data cluster. The witness provides quorum and must be placed outside the protected data-node pair so that it can arbitrate availability during host or site failure. If the proposed witness host resides inside the 2-node vSAN enabled cluster, it is not eligible and therefore does not appear as an available selection during the replacement workflow. Having a VMkernel adapter configured with vSAN traffic enabled is required for witness communication, so that is not the cause of the issue. Connectivity to the data hosts is also expected and does not make the witness invalid. A witness host already assigned to another 2-node vSAN cluster is not necessarily disqualified because shared witness configurations are supported within scale and sizing limits. The core eligibility issue is that the witness host cannot be a member of the same 2-node vSAN cluster that it is intended to witness. Reference topics: 2-Node vSAN Cluster, Witness Host Placement, Replace Witness Host, Shared Witness Support.