The Guest Network can only be enabled when the AirPort Extreme is configured to provide DHCP and NAT services.
If you already have another router on the network "upstream" from the AirPort Extreme, then the AirPort should be configured to operate in Bridge Mode to work correctly on the network. Unfortunately, the Guest Network feature cannot be enabled when the AirPort is in Bridge Mode. It's a Catch 22.
I am not recommending that you do this, but if you want to try to "break the rules" and run two devices both providing routing services on the network, you can try things out to see if they might work.
Assuming that you don't have DHCP IP address conflicts, you will always have a Double NAT condition when you do this. Double NAT will slow communications a bit and may produce other unpredictable results on the network. This "error" can range from a minor annoyance to a deal killer depending on what you are trying to accomplish and what other devices you might have on the network.
AirPort Utility does allow you to "ignore" the error, so the AirPort will display a green light even though the Double NAT condition exists.
If you decide to try this, let us know how things are working for you.