In a BO1 match of Riftbound, when are you allowed to side deck?

No — by default you can never sideboard in a BO1 match. Sideboarding is only permitted between games of a match (Rule 403.4), and a BO1 has only one game. Rule 403.5 additionally forbids using sideboard cards in the first game of a match.

The one exception: the head judge may opt into the altered BO1 setup process, which includes a single sideboard opportunity before game 1 begins (Rule 406.1.g.5). Unless the event specifically announced that change, your main deck stays locked.