Hi Pitawat,
Thanks for your patience.
We have got the below reply from internal team:
Reservations are not available for V5 SKu’s yet.
Creating new reservations is a time-consuming process and it will not be available until next year.
We recommend you to raise feedback over Azure Feedback Channel for Azure Database for PostgreSQL so that concerned team could consider this feature depending on the importance and up-votes received.
Hope this helps. Please let us know if you have further queries.
Thanks