s3ql_verify: 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 Debian possible default full paths are:

/usr/bin/s3ql_verify - from package: s3ql

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

Error in other languages:
s3ql_verify: bevel nie gevind nie
s3ql_verify: comandă negăsită
s3ql_verify: comando não encontrado
s3ql_verify: commande introuvable
s3ql_verify: command not found
s3ql_verify: käsku ei ole
s3ql_verify: không tìm thấy lệnh
s3ql_verify: komanda nerasta
s3ql_verify: Kommando nicht gefunden.
s3ql_verify: kommandot finns inte
s3ql_verify: komut yok
s3ql_verify: nie znaleziono polecenia
s3ql_verify: níor aimsíodh an t-ordú
s3ql_verify: no se encontró la orden
s3ql_verify: no s'ha trobat l'ordre
s3ql_verify: opdracht niet gevonden
s3ql_verify: parancs nem található
s3ql_verify: perintah tidak ditemukan
s3ql_verify: príkaz nenájdený
s3ql_verify: příkaz nenalezen
s3ql_verify: команда не найдена
s3ql_verify: командата не е открита
s3ql_verify: 命令找不到
s3ql_verify: コマンドが見つかりません