fsck.fat: command not found

Have You tried accessing the binary with the full path?

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 Debian possible default full paths are:

/sbin/fsck.fat - from package: dosfstools

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