Dear Roberto,
Thank you very much for your help. Your sharing made me feel more confident about my trial and errors. When I pair K375 using GUI interface, it never got to the point to type in the PIN. And the failed pairing entries always got stuck under the bluetooth menu
I found that I can use sudo bluetoothctl at the command line to manually start the pairing processes and monitored its progress. Somehow using the GUI version from the menu bar did not work for me.
I use the same approach to pair the keyboard and mouse. Now my desk space is no longer cluttered by 2 keyboards and 2 mouse
Thank you again for your help and assistance.
hmikelee