Diagnosis
That's the oven's way of telling you a door or safety switch isn't fully engaged. The most common culprit is the French doors not being perfectly aligned and latched.
How to Fix It
First, power it off and unplug it for a full minute. When you plug it back in, focus on the doors. Open them, then firmly close the left door first until you hear a distinct click. Immediately close the right door against it. You should feel and hear a solid seal. Try starting your cook cycle again. If it still beeps, check that nothing (like a crumb or the drip tray) is blocking the doors from shutting flush.
If That Didn't Work
The internal door switch itself might be faulty or have a loose connection. You can inspect it by looking at the door frame on the left side near the hinge area for a small plastic plunger. Make sure it moves freely when you press it. If it's stuck or broken, that's a job for customer service.
Heads Up
Never force the doors shut. If they don't close smoothly, something's in the way. Forcing it can crack the glass or damage the latch mechanism, which is a much bigger fix.