Diagnosis
The door latch mechanism is either misaligned or the spring-loaded hook is broken.
How to Fix It
Inspect the metal hook on the door and the strike plate on the oven body. Look for food grease buildup. Clean both thoroughly with isopropyl alcohol. Check if the door is sagging on its hinges. The hinges are not adjustable. If the hook is physically bent or the plastic housing on the door is cracked, you need a new door assembly.
If That Didn't Work
The microswitch behind the latch that tells the oven the door is closed may have failed. The oven won't start if this switch is bad, but a weak latch could fail to engage it properly. Testing requires a multimeter.
Heads Up
A door that doesn't seal causes massive heat loss and inaccurate cooking. It also poses a burn hazard. Do not use the oven until this is fixed.