Hi Erika,
thank you for your answer!
Here are my steps:
1. I've checked J21 and can confirm, there ist no contact to the upper board
2. I've downloaded your firmware file
2. I've pressed reset button and connected USB to the OpenSDA connector
3. I've copied the firmware file in the explorer drive BOOTLOADER
4. I've connected USB to KL25Z USB connector
Unfortunately, the status in the NXP GUI still keeps switching between "NOT CONNECTED" and "DISCONNECTED," and the Start button remains grayed out.