for example if using

bin/start-micro-quickstart

you would update

conf/druid/single-server/micro-quickstart/_common/common.runtime.properties

if that still doesn't work, make sure the function is spelled right? the most basic datasketches function is

APPROX_COUNT_DISTINCT_DS_HLL(expr)

— you can try that one just to confirm, like:

SELECT APPROX_COUNT_DISTINCT_DS_HLL(any_column)
FROM any_table

this is the simplest query that should work if the datasketches extension is loaded