I was trying to configure DirectAccess on UAG in a test environment, and kept getting the above error whenever I tried to activate my configuration.

The solution is simple. UAG needs two consecutive public IP addresses assigned on the external network interface. In a test environment, you would sometimes use a private IP range like 10.0.0.0 or 192.168.0.0 for the external interface, which is not supported. Also note, the UAG DirectAccess server cannot be behind a NAT.

Change the IP address on the external interface to a public IP address, and the error will go away!