Can I choose a unit in my hand to kill it with Heedless Resurrection or must the unit chosen be on my board?

To clarify how Heedless Resurrection works, let's look at the requirements for the two different actions it performs:

1. Killing a unit (The Cost): You must kill a friendly unit. According to the rules, "unit" refers to objects on the board unless specified otherwise (Rule 355.9.a.1). Therefore, you must choose a unit currently on your board to kill to pay the additional cost. You cannot choose a unit in your hand.
2. Playing a unit (The Effect): Heedless Resurrection specifically instructs you to play a unit from your trash. You cannot play a unit from your hand using this spell.

In summary:

• The unit you kill must be on your board (it is the cost).
• The unit you play must be in your trash (it is the effect).

You cannot use a unit from your hand for either part of this spell's requirements.