Cannot choose sub-directory when installing multisite on wp

My host does not allow wildcard sub-domains so I need to use a sub-directory system. When I go to enable sub-directory WP will not allow me to. I get the following message:

Welcome to the Network installation process!

Fill in the information below and you’ll be on your way to creating a network of WordPress sites. We will create configuration files in the next step.

Network Details

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