DHCP Reservations: "Failure! Please check your inputs."

Setting a static IP to a device in the router settings always gives the error  "Failure! Please check your inputs.".

Please don't tell me I need to enter a comment. I know how to google and I have seen that false advice many times. I enter a comment. I tried clearing the comment. I tried clearing it, restarting the router and reentering it. I tried changing the comment.

THE COMMENT HAS NOTHING TO DO WITH IT. ADDING, REMOVING, CHANGING, OR ANYTHING AT ALL WITH THE COMMENT DO NOT FIX THE PROBLEM.

I cannot believe for $1200/year this broken piece of garbage is what Xfinity tells us to use.