Hi. I've just gone to put in redirects before our site goes live tomorrow and had a fair bit of trouble putting in multiple redirects to the same target. The button was disabled as per the screenshot however luckily I figured out that if I load the screen again I can put the additional entries in.
Actually I don't need to refresh the page, all I need to do is clear out the target page and select it again and it works fine. There are no console issues either.