Comment 2 for bug 1560210

Not sure I get your problem. The error message is self-explicit : "Server disk was unable to be resized because: Resize to zero disk flavor is not allowed."

It seems the flavor you want for resizing your instance has a root_gb variable equals to 0. In that case, Nova doesn't accept to resize from an image-based instance to a volume-based one.

Setting the bug report as Invalid, but please move it back to New if you don't agree.