If you are using Administration.Account for your user data, then you need to make sure that the user has a role that has an access rule that allows for deleting. 

So, if your user has the Admininstration module role User assigned to it, this doesn't usually allow for deleting. You can change the access rights for this role by going into the module's Security options, selecting User, and checking "Allow deleting existing objects". You could also create a new role that allows deleting, while keeping the regular User role that doesn't. However, changing of this module may cause issues in the future if you want to upgrade it.

I hope this helps.

There are ways around this, for instance by marking an account for deletion and executing the actual deletion in a scheduled event. This has the additional benefit of allowing your user to reverse the process until the moment of deletion in case they made a mistake. 

Alternatively you can abuse features such as the "execute microflow as user" action, which allows you to let the deletion be executed by an admin account, but doing stuff like this means you have to be extra careful with security so it cannot be abused to delete other accounts.