When you schedule a Google Calendar event, make sure you set the correct sharing options.
Issue
When I schedule an event on a resource calendar, the event is rejected even though the calendar appears to be free.
Why is this happening? For someone in your organization to successfully schedule a resource, it must be shared as See all event details, or See only free/busy with calendar room booking allowed. If the resource has other settings, you might be able to view the resource, but the resource will always decline the event when you try to schedule it.
Resolution
Ask your Google Workspace administrator to change the resource's sharing options to See all event details, See only free/busy with calendar room booking allowed, or to give you specific permission to book the resource. For details, go to Share room and resource calendars.
More Information
If your Google Workspace administrator didn't explicitly set the sharing option for a resource, it inherits the default sharing option for calendars in your organization. For more information, go to Set Google Calendar sharing options.