ERROR: insufficient columns in the PRIMARY KEY constraint definition

I recently upgraded the database from PostgreSQL v9.6 to v11.7. we have some partitioned table with `inherence` and planning to migrate them to the `declaration`.
Table DDL:

CREATE TABLE c_account_p(    billing_account_guid character varying(40)  NOT NULL,    ingestion_process_id bigint NOT NULL DEFAULT '-1'::integer,    load_dttm timestamp(6) without time zone NOT NULL,    ban integer NOT NULL,    CONSTRAINT billing_account_pkey PRIMARY KEY (billing_account_guid, ban)) PARTITION by RANGE(load_dttm);

When I try the create table, it's throwing below error:
ERROR:  insufficient columns in the PRIMARY KEY constraint definitionDETAIL:  PRIMARY KEY constraint on table "l_billing_account_p" lacks column "load_dttm" which is part of the partition key.SQL state: 0A000

Is it mandatory/necessary that the `partition column` should be a primary key? cause if I  include `load_dttm` as `PK` then its working fine.
If the partition column should be supposed to be a PK, it's challenging to create a partition by range with the date column, cause the load_dttm column chances to have duplicate if data loaded `COPY`.

Could some please help me to understand this scenario?   https://dbfiddle.uk/?rdbms=postgres_11&fiddle=5000c4602c16350d77974868e62dc7c9  

Thanks.