The control board believes the door is locked even though it's not locked. There is a micro switch mounted on the door lock assembly that could have welded contacts signaling the control board the door is locked. You can access and check the switch operation by lifting the main top and removing access panel covering the lock assembly. See image below.
NOTE: Disconnect the power cord from the power source before removing the door lock assembly cover.
You can diagnose the failure simply by disconnecting the wires from the micro-switch and then see if the oven will operate in bake or broil. If it does, replace the latch assembly. If it continues to display "unlock door", the electronic oven control will need to be replaced. Note: The range will not go into the self clean mode if you don't replace the door lock assembly but you should be able to use the bake and broil.
The micro-switch is not available separately, the door latch assembly will have to be removed and replaced.
door lock assembly
is part number WB14T10040.