Follow these configuration to allow author add Rooms into their listing
Edit listing type post to select Hotel Room for child post type option then click Save Changes
Add/edit room submit fields
You also need add Calendar field to change room’s available dates
Add Listing Rooms field on listing submit/edit page
Then author can add/edit rooms for their listing
And to show rooms on single listing page. Edit listing type post -> Single tab -> Listing sub-tab to add Hotel Rooms element into layout builder.
And this is how rooms showing on listing page