Creating guest account on Time Capsule

Is your Time Capsule your primary router or do you have another AirPort acting as the primary router?

If you have another AirPort as the primary router, it's giving the Time Capsule an IP address in the range of 10.0.1.XXX. Your Time Capsule is in turn trying to distribute IP addresses in the range of 10.0.1.XXX to any devices connected to it. Therefore, the Time Capsule's external IP address (WAN IP address) is in the same range as the internal IP addresses it's distributing.

In order to set up a guest network, you need to set the Time Capsule as the primary router, or you would need to tell it to distribute IP addresses in a different subnet (10.1.1.XXX for example). The latter choice may result in a double NAT error, and any devices connected to your Time Capsule will not be able to communicate with devices connected to your primary AirPort router.