
I didn't have any issues adding the 300 set to my cart. They are not out of stock.
Try another browser, or check for ‘blocking’ popups, etc.
Or call Sonos Support directly to discuss it, they’ll take your order, I suspect.
As @Airgetlam suggested try another browser and/or clear the cache (history) on whatever browser you’re using.
The link below is to Sonos sales. They can take your order over the phone.
https://www.sonos.com/en-us/contact