Can you use a Seal for Deflect cost?

Yes, you can use seals to pay for Deflect costs.

According to the official FAQ and Rule 735.1.c.1:

• Deflect adds a mandatory additional cost to the Power cost of a spell or ability.
• The Power used to pay a Deflect cost can be of any Domain (it does not need to match the color of the spell or the target).
• Since seals generate Power, they can be exhausted to satisfy this additional cost.

Important Nuances:

• Power vs. Energy: Seals only generate Power, not Energy. You cannot use a seal to pay for the numerical Energy cost of a card, only the Power requirements (including those added by Deflect).
• Triggered Abilities: If a triggered ability (like a "When I'm played..." effect) targets a unit with Deflect, you can choose to refuse to pay the cost. If you do, the ability simply fails to finalize and is removed from the chain (Rule 392.2).