Couchbase for Ubuntu 16.04 LTS
According to Couchbase's issue tracker at https://issues.couchbase.com/browse/MB-19272, it seems that Couchbase is still in the process of making Couchbase Server work on Ubuntu 16.04, so you are out of luck for the time being.
These instructions worked for me.
Essentially download the .deb for the 14.04 version and install it via dpkg