From what i remember, you either need to log in in the obs browser, or set the stream to public. The problem is likely, that you use the link from your dashboard which includes information that is not visible to others, or at least shouldn't be.
Try using the link from your watch page instead.
As far as i remember the chat is not usable for others than yourself when you stream a sheduled event. In this case it can be fiddly as you need to login with the obs browser before opening the chat link.
This should work when you open youtube in the browser source, then right-click->interact and login to the site. After that you should be able to open the chat.