A resource must be chosen.

A Resource is the item being booked. You can use this module to manage booking of multiple objects, each of their own type and at their own physical location etc. After digging through the code I noticed that resource handling is probably meant to be (atleast all GUI stuff) handled by the "resource-type plugins", of which atm there is only one: publicbookings.

Resource managment is not available unless you also enable the publicbookings module. You can then find it under admin/content/publicbookings/resource (appears as a sub-menu below Public bookings under admin/content).
I have yet to find a link to the page where one defines the availability for a resource though. (Not the default status, but the actual periods when a resource is available or unavailable). I ended up hacking my own link into the resources list, it goes to /admin/content/publicbookings/resource/RESOURCE_ID/availability/list. Without the ability to clearly define resources as strictly unavailable by default (when creating/editing it), as well as defining when the resource is actually available, would make this completely useless to me hehe.