Question: 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?

Answer: 
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' );

```
[This thread](https://premium.wpmudev.org/forums/topic/enabling-network-because-your-install-is-not-new-the-sites-in-your-wordpress-network-must-use-sub-domains) might be helpful.

In essence: try to delete sample post and page.