That default command screen is used to run pre-existing programs that you have saved on your turtle. The reason it's saying that is because there is no program on that turtle called turtle.attack().
You need to create a program, and then run it by entering the name of the program on the default screen. If I remember correctly, you make a new program by going like...
and replace 'name' with whatever program name you want.
Of course, I'm probably one of least informed people on these forums in regards to ComputerCraft. Someone more educated on that particular mod will inevitably stumble across this thread.