rsstail: 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/rsstail - from package: rsstail

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

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