To enable the option to allow users to delete their accounts, please go to Settings>General>General Settings. Then look for the Account Deletion and put a mark on it. Once done, click Save Settings to apply changes. If you are using the BuddyBoss App as well, this is the same option that you need to enable. 

Please be informed that this display will not be showing for admin accounts. 

Here is the sample result:

Here is a sample result for the BuddyBoss App: