I can not access my guest network

You may not have the correct type of "modem" to allow the guest network to function correctly. The guest network will not operate correctly if you have a modem/router or gateway type of device that looks something like this:

This type of device is both a modem and router in one package.

In order for the guest network to operate correctly, the AirPort router needs to connect to a simple modem, like this example:

Which type of "modem" do you have?

If you are not sure, post back with the make and model number of the device that you call your "modem" and we'll check on that for you.

It’s easy to allow guests to use your Internet connection without sharing your password or giving them access to the rest of your network. Simply enable the guest networking feature using AirPort Utility and create a separate Wi-Fi network just for your guests. You can set up the guest network with a different password or with none at all. Your primary network, including your printer, attached drives, or other devices remains secure.

An apple article is there : AirPort base stations: About the Guest network feature - Apple Support

Despite what the Apple article might say, the network will not operate correctly if the user has a modem/router or gateway device on his network.