Quite simply - your network doesn't support video calling (not all networks support the feature).
Have a read of this (copied from elsewhere):
To make a video call you need to check the following things first:
You have a good 3G or 3.5G signal, without that video calls are not possible. If you don't get 3G contact your network to check coverage in your area or if you have a compatible sim card.
Your network supports video calling, not all networks support the feature.
The other persons phone and network supports video calling and they are in a good 3G/3.5G signal area. In some countries you can only video call someone on the same network as you.
Don't forget to check the cost involved!
Once you've done that go into your contacts, select the person you want to call and select the video call option.
If everything is ok then it will work, if not it will either fail or make a standard voice call instead.