A System Administrator attempts to deactivate a user but receives the error message "You CANNOT deactivate this user". What is the reason the Administrator is unable to deactivate the user?

Users can be deactivated regardless of role assignment, as long as they don't hold critical system positions that prevent deactivation.

Being logged into Salesforce does not prevent a user from being deactivated; they will simply be logged out upon deactivation.

Salesforce prevents deactivating a user who is set as the default owner for leads or cases because this would leave these records without a designated owner for automatic assignment rules. To deactivate such a user, an administrator must first reassign the default lead and case ownership to another active user in the organization's settings.

While users owning open opportunities can be deactivated, the system will prompt for reassignment of these records upon deactivation, but it does not prevent the deactivation itself.