The version of CQLSH you are using is utilizing CQL 3.0.0 and distinct is not available until 3.1.1.
From the manual
3.1.1
SELECT statement now allows listing the partition keys (using the DISTINCT modifier). See CASSANDRA-4536.
The syntax c IN ? is now supported in WHERE clauses. In that case, the value expected for the bind variable will be a list of whatever type c is.
It is now possible to use named bind variables (using :name instead of ?).