If a card with flashback is put into your graveyard during your turn, you can cast it if it's legal to do so before any other player can take any actions.

"Flashback [cost]" means "You may cast this card from your graveyard by paying [cost] rather than paying its mana cost" and "If the flashback cost was paid, exile this card instead of putting it anywhere else any time it would leave the stack."

You must still follow any timing restrictions and permissions, including those based on the card's type. For instance, you can cast a sorcery using flashback only when you could normally cast a sorcery.

To determine the total cost of a spell, start with the mana cost or alternative cost (such as a flashback cost) you're paying, add any cost increases, then apply any cost reductions. The mana value of the spell is determined only by its mana cost, no matter what the total cost to cast the spell was.

A spell cast using flashback will always be exiled afterward, whether it resolves, is countered, or leaves the stack in some other way.

You can cast a spell using flashback even if it was somehow put into your graveyard without having been cast.

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 their 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 their 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.

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