Why you see the "co" respawn error...
Your version of Linux wants to see the "ttys0" or "S1" format to identify the default serial console device. Maybe you upgraded to a secure version... maybe you switched distributions... or maybe someone inserted code from some Q and A website without understanding it. But, the reply below was correct (put a hash mark at the beginning of the line, to comment it out, because it's not doing anything useful on your current version of Linux).
Look at the other lines in the file. Do you have other lines with the agetty command? That's probably doing what you need already, if your serial console is working.
In my case, I had the old style, but invoking the secure version of the OS required a different syntax. I kept the old one, commented out and added further comment, so that the next sysadmin won't remove the comment mark by mistake. Part of my inittab look like this...
# Run gettys in standard runlevels
S1:12345:respawn:/sbin/agetty -L ttyS1 9600n8 vt100
#co:2345:respawn:/sbin/agetty ttyS0 9600 vt100-nav #Line above used instead of this line for console redirection.
Hoping you have happy consoles,