1024x768 suggest a really ancient monitor which MorphOS cant regognize.
Go to preferences->monitor (not screen).
On the top you will see something like "Monitor-38KHz"
Right to it is a gadget "edit", hit that and press "ok" in the requester that pops up.
Here you will to choose a fitting monitor, if yours is not listed try to figure out it's data and choose the according standard one.
Now 1024x768 sounds like either a 17" CRT or 15" LCD, you might wanna try "Monitor-64KHz".
You'll need to save and reboot to activate the new settings.
Afterwards you should be able to select a proper 1024x768 screenmode.
