A recent Outlook user, trying to book a non-recurring meeting, received this error: "Giant Conference Room has declined your meeting because it is recurring. You must book each meeting separately with this resource." But, it was non-recurring!!!
This error comes about when multiple users have the same resource open, in this case a conference room, and make edits causing the Free/Busy information to become corrupt.
In order to fix the issue, just change one of the scheduling settings within Outlook > Options > Calendar > Resource Scheduling..., such as "Automatically decline recurring meeting requests," click OK, then change it back. It'll cause the Free/Busy data to rebuild and scheduling may once again take place.