MultiSite Network Setting Snafus

Greetings. Am trying to complete the installation of Multi-Site.

I read: We recommend you change your siteurl to before enabling the network feature. It will still be possible to visit your site using the www prefix with an address like but any links will not have the www prefix.

I went into general settings and attempted to remove the http://www, but when I tried to save it said “not a valid URL.” This was both in site URL and WP URL.

Am I in the wrong admin panel?

Please help. TYVM.

Cheers. – Frank