Block a range of external IP addresses

Hello.  We have been subject to a DDoS attack over the last day, and I would like to block all traffic coming from a range of IP addresses in Brazil.  (177.36.1.1 to 177.36.255.255).

We have a CBRT-2 Gateway.  I have logged into the Gateway.  I believe i am supposed to go to Port Management and set it to allow all traffic except for defined exceptions and then set up the range of addresses to block as an exception.

The problem is that when I try to enter the range, I get the error "Start IP is in range of neither true static IP subnet nor additional public subnet."  I don't see anywhere in the interface to either set these subnet ranges or see what they are.

What am I doing wrong?  Thank you.