When you edit a custom view on your lists or calendar, you may see a message that your view is owned by "System" and the option to make it private is greyed and can't be checked.
Private views can be viewed only by the user who created them - the view's "owner". We added the option of a private view in January 2019 and any views created prior to that release did not have an owner. If we allowed these views to be made private, they would not be visible to any users on the account.
To make the view private, you will need to save the view as a new one and then remove the original. You'll be able to do this in just a few clicks. The video below will walk you through this.