You don’t have to target any cards when you cast Memory’s Journey, but you must target a player.

If the player is an illegal target by the time Memory’s Journey resolves, the spell will have no effect, even if the cards are still legal targets. This is because a spell can’t make an illegal target (the player) perform any actions (such as shuffling his or her library).

Any of the targeted cards that are illegal targets by the time Memory’s Journey resolves aren’t shuffled into their owner’s library.

If no cards were targeted by Memory’s Journey or if all the targeted cards are illegal targets by the time Memory’s Journey resolves, the targeted player will still shuffle his or her library.

If you cast Memory’s Journey with flashback, it won’t be in the graveyard when you choose targets. It can’t target itself.