Comment 2 for bug 1836526

There are buildd images for s390x already available.
https://cloud-images.ubuntu.com/buildd/daily/bionic/20191209/

I ran into the same issue building microstack on s390x.

~/microstack$ sudo snapcraft --use-lxd
The LXD provider is offered as a technology preview for early adopters.
The command line interface, container names or lifecycle handling may change in upcoming releases.
Launching a container.
An error occurred with the instance when trying to launch with 'LXD': The requested image couldn't be found.
Ensure that 'LXD' is setup correctly and try again.