s3ql_oauth_client: 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:

/usr/bin/s3ql_oauth_client - from package: s3ql

You may install the required package with command apt-get install {package}
Error in other languages:
s3ql_oauth_client: bevel nie gevind nie
s3ql_oauth_client: comandă negăsită
s3ql_oauth_client: comando não encontrado
s3ql_oauth_client: commande introuvable
s3ql_oauth_client: command not found
s3ql_oauth_client: käsku ei ole
s3ql_oauth_client: không tìm thấy lệnh
s3ql_oauth_client: komanda nerasta
s3ql_oauth_client: Kommando nicht gefunden.
s3ql_oauth_client: kommandot finns inte
s3ql_oauth_client: komut yok
s3ql_oauth_client: nie znaleziono polecenia
s3ql_oauth_client: níor aimsíodh an t-ordú
s3ql_oauth_client: no se encontró la orden
s3ql_oauth_client: no s'ha trobat l'ordre
s3ql_oauth_client: opdracht niet gevonden
s3ql_oauth_client: parancs nem található
s3ql_oauth_client: perintah tidak ditemukan
s3ql_oauth_client: príkaz nenájdený
s3ql_oauth_client: příkaz nenalezen
s3ql_oauth_client: команда не найдена
s3ql_oauth_client: командата не е открита
s3ql_oauth_client: 命令找不到
s3ql_oauth_client: コマンドが見つかりません