rsnapshot-diff: 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/rsnapshot-diff - from package: rsnapshot

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

On Debian possible default full paths are:

/usr/bin/rsnapshot-diff - from package: rsnapshot

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

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