BR200 /23 subnet issue

I'm having trouble making new subnets with a mask of 255.255.254.0. It keeps giving me an error "ip address existed.", but I can't find that phrase word for word anywhere. And it doesn't really make sense to me... LAN 1 is 192.168.1.1 (or 0.1 like the photo) and LAN 2 is 192.168.10.1, they're nowhere close to overlapping. Can someone explain what I'm doing wrong? I can set up a network like this just fine on my Ubiquiti equipment, so I'm wondering if there's something Netgear specific I'm missing.