Building classic snap on Ubuntu Core

Hi;

I wanna build my snap package on Ubuntu Core. I have already done that successfully on Ubuntu using Snapcraft and I have snapcraft.yaml file. So I run these commands on Ubuntu Core:

$ sudo snap install classic --edge --devmode
$ sudo classic

Then I should be able to run classic ubuntu commands. I can install apt packages, but when I want to install Snapcraft, I got this error:

(classic)<my_user>@localhost:~$ snap install snapcraft
error: snap "snapcraft" requires classic confinement which is only available on classic systems

So how I can I install Snapcraft?

Thanks in advance.

Bests,
Vahid