You must create message ordering pub/sub and Cloud function manually.
Fisrt, Create a pub/sub topic, and then create a subscription that subscribes pub/sub topic with
Second, Create a Cloud function that will serve ordered pub/sub messages.
Last, back to the pub/sub subscription, Edit delivery type to push and specify your cloud function endpoint.
So final diagram is like below.
Publisher -> Pub/sub topic -> Pub/sub subscriber -> Cloud function
You tried to make a connection Pub/sub topic with Cloud function directly.
But for message ordering, Pub/sub needs topic -> subscriber connection.
So only pub/sub topic -> pub/sub subscriber -> Cloud function connection can delivers ordered messages to your function.