Hello,
sorry for my previous confusing answer I have overlooked the Z letter. Unfortunately the MK20DX256VLK10 is not supported yet. I have also discussed the workaround with developers and they recommended to select MK20DN512VLK10 and tweak the flash settings.
We should be able to support this part in update 2 for CW V10.3. This release should be published in the middle of Q2.
best regards
Vojtech Filip
Processor Expert Support Team