screen: 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/screen - from package: screen

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

On Debian possible default full paths are:

/usr/bin/screen - from package: screen

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

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