How do I fix a hole in a stone firepit?

It suppose to be there...air intake, air hole for air flow to keep fire going; are you sure it's not part of the pit probably should not block it. "Every fire pit needs air flow to provide oxygen to flames. This becomes more of an issue when you light a fire contained within an aboveground walled pit. Place one 2-inch hole every 24 to 36 inches around the base of the pit. Keep holes cleared of ash and other debris to ensure adequate air flow." another "To work correctly, the fire pit must have a constant supply of air. If the air holes or air intake vent have been plugged, or don't exist at all, you may have a hard time getting a fire to burn. "

No, I’m sure it’s not supposed to be there. I understand how fire pits work. It’s cracked. I have a picture from the evening. I will take a better one later today.

Here is the evening one

What put the hole in the bottom of your firepit? I would check into that before I tried to fix it otherwise your patch may just go the way of the original concrete. There is a concrete patch that you can purchase but I don't know if that is fire/heat proof. I have used it on driveways and walkways in full sun and it has held up but not under a flame.

I would stuff some aluminum foil in the hole as a backing. Then fill with a concrete patch make for high heat.