They are provisioned by a HRIS integration
If your people are provisioned by an external HRIS, you will need to take steps to remove them from the HRIS first.
You can tell if someone is provisioned by a HRIS integration by looking at the icon in the "Managed by" column of the Company view.
Any changes should be reflected in Frankli the next time it syncs with the HRIS.
They have had their access revoked
When a person has had their access revoked, they cannot be archived.
You will need to temporarily restore access for the archive option to appear next to them in the Company view.