Serial console not working for A1.Flex

I followed all of the instructions from https://docs.oracle.com/en-us/iaas/Content/Compute/Tasks/enablingserialconsoleaccess.htm in regards to enabling the serial connection but can't get it to work.

Attempting to start it manually with `sudo systemctl stop getty@ttyS0` results in failure:

 /dev/ttyS0: not a tty

Thanks for your help in advance!