xsel: 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/xsel - from package: xsel

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

On Debian possible default full paths are:

/usr/bin/xsel - from package: xsel

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

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