Members need to be deactivated before they can be deleted.
Any members no longer allowed to use the platform but who do not need to be deleted altogether can be deactivated.
This can be useful for external project managers who are due to return at a later date but do not need access for now, for instance.
You can find a list of deactivated members under the Deactivated members tab.
You can then delete or reactivate the member.