WordPress Multisite subdirectories not allowed on new install?

I’ve just installed a fresh install of WordPress and before doing anything else I’ve tried to setup a multisite, which I would like to do with subdirectories.

But when I click on Network Setup I get the notification:

Because your installation is not new, the sites in your WordPress
network must use sub-domains. The main site in a sub-directory
installation will need to use a modified permalink structure,
potentially breaking existing links.

Can anyone tell me what I’m doing wrong, or what I need to change to allow subdirectories?

You don’t need to do anything just past this code in your activated theme’s functions.php

add_filter( 'allow_subdirectory_install', '__return_true' );