cu: /dev/ttyS0: Line in use

cu: open (/dev/ttyS0): Permission denied
cu: /dev/ttyS0: Line in use

There is probably no real usage of the line, but a permission issue. quick and dirty way to test for me was to execute:
ls -la /dev/ttyUSB0
sudo chmod 666 /dev/ttyUSB0


and retry cu. If it starts working, you need to take care of the respective udev file and the user permissions/groups. For my device it looked like this (being member in plugdev group):
> cat /etc/udev/rules.d/42-CP210x.rules 
ATTRS{idVendor}=="10c4", ATTRS{idProduct}=="ea60", SUBSYSTEMS=="usb",
ACTION=="add", MODE="0666", GROUP="plugdev"
source:
https://superuser.com/questions/794309/linux-tells-me-a-serial-port-is-in-use-but-it-isnt