df: 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:

/bin/df - from package: coreutils

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

On Debian possible default full paths are:

/bin/df - from package: coreutils

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

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