When you schedule a Google Calendar event, make sure you set the correct sharing options.
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 anyone in your organization to successfully schedule a resource, it must be shared as See all event details with everyone in the organization. If the resource has other settings, you might be able to schedule the resource, but the resource will always decline the event.
Ask your administrator to change the sharing options for the resource to See all event details or to give you specific permission to book the resource. Your administrator can find more information about Sharing calendar resources.
If the administrator of your domain did not explicitly set the sharing option for a resource, it will inherit the default sharing option for calendars in your organization. For more information, administrators should see Set Calendar sharing options.