To a new K64 chip which still didn't download the firmware, the constantly resetting in the reset pin is correct, after you download the code, it will be high.
Now, at first, I highly recommend you find a JLINK, and use the JLINK to connect your own designed board, use the JLINK command to check whether you can find the ARM core or not?
An example for you:
Please check it at first, if you can't find the ARM core, it means your hardware still have issues.
If you can find the ARM core, then you should can debug the code.
BTW, your hardware: NMI pin need to be pull up with 4.7K to 10K resistor, reset pin, 4.7K to 10K pull up, and 0.1uf capacitor to ground.
SWD_DIO pull up 10K.
Any updated information, just kindly let me know.