Can you target a card with deflect without paying the deflect cost, and what happens if you don't pay?

Ruling: You cannot legally play a spell targeting a card with deflect without paying the deflect cost. Deflect is a mandatory additional cost that must be paid when you play the card. If you cannot pay the deflect cost, the play is illegal and gets undone - the card returns to your hand.

Sequence:

• When you play a spell targeting a card with deflect, you determine the deflect cost at that time
• You must pay the deflect cost as part of playing the card
• If you cannot pay the cost, the game enters an illegal state
• The actions are undone and the card goes back to your hand

Nuances:

• Playing a spell that gets undone due to inability to pay deflect does not count as playing a spell for effects like Ravenbloom Student, since the play was illegal and reversed
• There is some uncertainty about whether you are forced to recycle runes to pay deflect if you don't already have floating power, but regardless, if you don't pay, the play is illegal and gets undone