How do I install/enable the uuid-ossp extension on Postgres 9.3?
You seem to have two versions (9.6 and 9.3) installed (because the psql version is 9.6).  Because the postgresql-contrib package automatically defaults to the currently supported PostgreSQL database contrib package (as noted if you do apt-cache show postgresql-contrib), you'll have to install the contrib package for 9.3:

apt-get install postgresql-contrib-9.3