co: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/co - from package: rcs

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/co - from package: rcs

You may install the required package with command apt-get install {package}

Error in other languages:
co: bevel nie gevind nie
co: comandă negăsită
co: comando não encontrado
co: commande introuvable
co: command not found
co: käsku ei ole
co: không tìm thấy lệnh
co: komanda nerasta
co: Kommando nicht gefunden.
co: kommandot finns inte
co: komut yok
co: nie znaleziono polecenia
co: níor aimsíodh an t-ordú
co: no se encontró la orden
co: no s'ha trobat l'ordre
co: opdracht niet gevonden
co: parancs nem található
co: perintah tidak ditemukan
co: príkaz nenájdený
co: příkaz nenalezen
co: команда не найдена
co: командата не е открита
co: 命令找不到
co: コマンドが見つかりません