"This cake is not yours" sounds more native. I can't tell if it's referring to a whole cake or a portion, and it doesn't make sense that a whole cake would be a portion, yet it hasn't qualified that 'this cake' is a portion of it's own.
I agree with the other comments. This English sentence makes no sense. Two choices
This piece of cake is not yours. Or
This is not your piece of cake.
Thanjs Duolingo for getting so much right....my comments are just to help improve the site