Does convoke let you tap a creature that just entered the battlefield this turn?

Convoke (CR 702.51) is a keyword ability that allows you to tap creatures you control as part of paying the cost of a spell. Each creature you tap reduces the spell's cost by either one generic mana or one mana of that creature's color. Crucially, convoke does not require the creature to have been under your control since the beginning of the turn.

The normal rule about summoning sickness (CR 302.6) only prevents a creature from attacking or from being tapped to pay a cost that uses the tap symbol (T) in an activated ability. Convoke is neither of those things — it is a special alternative cost mechanism applied during the casting of a spell, not an activated ability of the creature itself.

CR 702.51b specifically states that tapping a creature for convoke is done as part of casting the spell, and CR 302.6 confirms that summoning sickness only restricts attacking and using activated abilities with the tap or untap symbol. Tapping for convoke falls outside both restrictions.

Concrete example: You play a Forest on your turn and then cast a Llanowar Elves, which enters the battlefield. Later that same turn, you cast Chord of Calling with convoke. You may tap the Llanowar Elves (which has summoning sickness) to help pay for Chord of Calling, reducing its cost by one green mana, because you are not using an activated ability — you are tapping it as part of convoke.

So as long as the creature is untapped and under your control at the time you cast the convoke spell, it is eligible to be tapped for convoke regardless of when it entered the battlefield.