user-clip: 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/lib64/piglit/bin/user-clip - from package: piglit

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

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