Can you tap a creature for convoke and also use it to attack in the same turn?

Convoke (CR 702.51) lets you tap creatures you control as you cast a convoke spell to help pay its mana cost. When you tap a creature this way, it becomes tapped — and a tapped creature cannot be declared as an attacker (CR 508.1a requires each chosen attacker to be untapped).

The timing also matters. You cast spells during your main phase, but you declare attackers during the combat phase's Declare Attackers step, which comes after your pre-combat main phase. So if you tap a creature for convoke in your main phase, it will still be tapped when the Declare Attackers step arrives and cannot attack.

The reverse is also true: if you attack with a creature (tapping it during the Declare Attackers step), it is now tapped for the rest of the turn, so you cannot use it for convoke on a spell cast afterward in your second main phase either.

Concrete example: You control a 2/2 Grizzly Bears and want to cast a convoke spell costing {4} by tapping it. You tap Grizzly Bears, reducing the cost by {1} (one generic mana). Grizzly Bears is now tapped. When combat begins, you cannot declare Grizzly Bears as an attacker because it does not satisfy the untapped requirement of CR 508.1a.

In short, tapping a creature for convoke and attacking with it in the same turn are mutually exclusive — you must choose one or the other.