I cast Spiritual Weapon to spawn a sword beside an enemy.
Can I use my action to ready my Spiritual Weapon to attack (for example, when the enemy tries to flee - not Opportunity Attack) when a condition is met?
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Both the summoning and controlling of the spiritual weapon use your bonus action:
As a bonus action on your turn, you can move the weapon up to 20 feet and repeat the attack (PHB 278)
While the PHB is a bit unclear on this, Jeremy Crawford stated here:
A bonus action can be taken only on your turn. You, therefore, can't use Ready with a bonus action.
Spiritual weapon states:
When you cast the spell, you can make a melee spell attack against a creature within 5 feet of the weapon. [...] As a bonus action on your turn, you can move the weapon up to 20 feet and repeat the attack against a creature within 5 feet of it.
You can’t ready a bonus action and bonus actions can only be taken on your turn, therefore you can’t ready an attack with spiritual weapon and it’s specifically stated that the first attack is made when you cast the spell.