User is trying to delete some accounts in the Chart of Accounts screen, but gets an error message that system accounts cannot be deleted. User wants to use different default accounts and not the ones that come with Core.
You cannot edit or delete the default system accounts in Core that are set in Global Settings > Accounting. However, if you create your own accounts and set them as the default system accounts in Global Settings, you are able to delete them or make them inactive as they are no longer default system accounts (provided there are no transactions associated with them). Check out this article for more information on editing system accounts in Core.