why is the add channel button grayed out?

It wasn't grayed out for me and I was able to add it.  However, it has issues with the latest Roku v12.5 software.

The only reason I can think of for it to be grayed out is that you don't have a Roku device linked to your account that supports it, but that seems very unlikely.  What's the model number of your Roku device displayed under Settings->System->About?

My only other thought is that it's some kind of region issue, but it shouldn't even show up if it's not available in your region.

Try clearing your browser cookies, using a private or incognito browser window, a different browser altogether, or a different device altogether.

You're probably viewing the web page without logging into your account.