i have to write a canon at the octave in a major scale

does that mean i'm limited to one scale position within each voicing? or that there has to be a perfect octave between the two lines?

I think it just means you have to start the canon an octave higher from the voice that entered before.
if u start with a theme with voice1 A B C , you just make the voice2 start with A B C an octave hier