Sorry, no G series can run any version of OSX past 10.5.8, higher requires an IntelMac.
So we know more about it...
At the Apple Icon at top left>About this Mac, then click on More Info, then click on Hardware> and report this upto but not including the Serial#...
Model Name: eMac
Model Identifier: PowerMac6,4
Processor Name: PowerPC G4 (1.2)
Processor Speed: 1.42 GHz
Number Of CPUs: 1
L2 Cache (per CPU): 512 KB
Memory: 2 GB
Bus Speed: 167 MHz
Boot ROM Version: 4.9.2f1
If you don't know the model, find the Serial# & use it on one of these sites, but don't post the Serial# here...
How to find the serial number of your Apple hardware product...
To use your Time Capsule for TIme Machine Backups, you will need to upgrade your OS to 10.5 Leopard. That is the only version that supports Time Machine and will run on a G5.
Best place for Time Capsule/AirPort issues is probably here:
And to find out what software you need to administer an AirPort Base station or Time Capsule:
Looks like you're going to need Leopard to administer the Time Capsule, unless you have another machine that can run the software.