fsck.exfat: 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.exfat - from package: exfat-utils

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