Naive question; when booking on line how do you reserve bike slot?
Only some operators' websites allow you to do this. Eastcoast is one. Southern sounds like another.
Beyond that, the ticket has to be on an operator that allows cycle reservations. The back-end will often fail ungracefully claiming that "there are no bike spaces available on your chosen service" when in fact the operator doesn't require reservations at all. More frustratingly, this will often happen when a journey includes multiple operators, some of whom don't - eg. for Brighton to Birmingham, it'll fail because Southern don't do reservations, and not allow you to book one for the Virgin segment.