Hi Antonio,
For KL02Z32VFG4, ADC channel 6 is reserved, ADC0_SE6 is ADC channel 9, please check the following picture from the reference manual, page 63.

So, I think you configure the wrong ADC channel, you should choose SC1n[ADCH] = 01001.
Wish it helps you!
If you still have question, please contact me!
Have a great day,
Jingjing
-----------------------------------------------------------------------------------------------------------------------
Note: If this post answers your question, please click the Correct Answer button. Thank you!
-----------------------------------------------------------------------------------------------------------------------