Turning on Federation

So I have had my Lemmy up and running for a few weeks now and I thought I would like to enable federation.

I saw the note to insert this code https://github.com/lemmynet/lemmy/blob/main/config/config.hjson#L64

And I saw somewhere we could do it through the interface.

So I click on the settings cog and insert it into the Admin Settings - Site Configuration box and click save.

But I always get an error message that says "Couldn't update this site"

What am I doing wrong? How do I enable federation?

Thanks is advance for your help.