Hi Nick,
Thank you for your updated information.
OK, then you need to use the external debugger to firmware your on board K20 chip again.
The K20 chip firmware can be found from this link:
OpenSDA Serial and Debug Adapter | NXP
About the K20 chip, I find from your picture, you are just select the MK20 chip, but you need to choose the device as MK20XXX_(ALLOW SECURITY). Then after you connect it, in the JLINK commander, you can use this commander to do the mass erase:
unlock kinetis
Then you can firmware your chip again.
Please try it again.
If you still have question about it, please kindly let me know.
Have a great day,
Kerry
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