Diagnosis
That's normal and not a malfunction. It's usually one of two things: manufacturing oils burning off from the heating element (first use) or moisture/cleaning residue burning off from inside the chamber (after cleaning).
How to Fix It
For a new unit or after a wet clean: 1) Make sure the basket and the unit itself are completely dry. 2) Run the air fryer empty at 400°F for 10-15 minutes. Do this in a well-ventilated area, maybe with a fan on or a window open. You'll likely see some smoke and smell a hot, oily odor. This is the burn-off process. Let it complete its cycle and cool down. The smoking should not happen during normal use after this.
If That Didn't Work
If it smokes during normal cooking, you likely have excess oil or grease dripping from fatty foods (like bacon or sausage) down onto the heating element. Use a trivet or put a slice of bread in the bottom of the basket under very fatty foods to catch drips.
Heads Up
Never use the air fryer for the first time or after a deep clean without doing this burn-off cycle. It will set off your smoke alarm. Also, always clean the basket after cooking greasy foods to prevent grease buildup, which is a fire hazard.