If my unit has a mandatory effect to target an enemy unit and the enemy unit has deflect, do I have to use the effect, and if I do not have enough to pay the cost for deflect, what happens?

To answer your question: No, you do not have to use the effect if you cannot pay the Deflect cost.

Here is the breakdown of how this interaction works based on the rules of Riftbound:

1. The Nature of Deflect

According to Rule 735.1.d, the Deflect keyword imposes a Mandatory Additional Cost on any spell or ability that chooses the permanent with Deflect.

2. Finalizing the Ability

When you attempt to play an ability that targets an enemy unit, you must go through the process of "finalizing" that ability on the chain. This includes choosing your targets and paying all costs associated with the ability (including any mandatory additional costs like Deflect).

• If you cannot pay the cost: You cannot complete the process of playing the ability. Because you cannot pay the mandatory additional cost required to target that specific unit, you are unable to choose that unit as a target.
• If the ability is mandatory: If the ability's text says you "must" target a unit, but you cannot pay the Deflect cost to target the only available enemy unit, you simply cannot play the ability. You cannot be forced to perform an action that you cannot legally pay for.

3. Summary of the Sequence

1. You attempt to activate your unit's ability.
2. You identify the target (the enemy unit with Deflect).
3. The game checks for mandatory additional costs. It identifies the Deflect cost.
4. You check your available Power.
5. If you have the Power: You pay the cost, the ability is placed on the chain, and it targets the unit.
6. If you do not have the Power: You cannot pay the mandatory additional cost. Therefore, you cannot choose that unit as a target. Since you cannot choose a legal target, you cannot play the ability.

In short: You are never forced to pay a cost you cannot afford. If the only legal target for a mandatory effect is protected by Deflect and you cannot pay the Deflect cost, you are effectively prevented from playing that ability.