Libicu55 package missing

I’m working on a snap package for a .NET program. After attempting to repackage this snap multiple different ways, I always end up back at the same error regarding a missing libicu55 package:

Failed to fetch stage packages: Error downloading packages for part 'brain': The package 'libicu55' was not found..

I’ve tried manually listing that package in the stage-packages section of the yaml file.

After coming across this syntax in the snapcraft.yaml in the snapcore github repo, I tried referencing a direct link to the libicu55 package.

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I’ve also manually installed ilibicu55 via apt-get locally onto my machine.

Any thoughts on what may be causing this?

Which Ubuntu Core version your snap is targeting on? libicu55 package is only provided by Ubuntu 16.04.