You should take a step back and read the basic documentation on PG administration. For instance: https://www.postgresql.org/docs/16/creating-cluster.html
We (at least I) have been assuming that you wanted to create a new database cluster, to run a server against that storage, using initdb. Using an already running server to put data elsewhere, using createdb is different, and requires that you create a tablespace first: https://www.postgresql.org/docs/16/manage-ag-tablespaces.html