Usually it would be because there aren't Ceph packages, or a new enough version of Ceph to be able to build the -ceph, -rgw, -rados-urls, and/or -rados-grace packages.  Xenial (16.04) is the oldest version of Ubuntu that we build for, and the Ceph version that's available for that is quite old.

I'll take another look, but I'd suggest you consider using a newer Ubuntu, e.g. 18.04 or even 20.04.

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Kaleb