Diagnosis
Grease and food particles have hardened in the basket handle's slide mechanism, jamming the release button.
How to Fix It
Let the unit cool completely. Press the basket release button firmly while wiggling the basket side-to-side. If stuck, submerge just the basket and handle assembly in hot, soapy water for 15 minutes. Scrub the handle's sliding tracks with a bottle brush. Work the button repeatedly underwater to loosen debris.
If That Didn't Work
The plastic release latch inside the handle may be broken. Inspect it. You may need to order a replacement basket from Instant Pot.
Heads Up
Never use metal tools to pry the basket out. You can scratch the non-stick coating. Clean the handle mechanism weekly to prevent this.