liferea-add-feed: 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/liferea-add-feed - from package: liferea

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

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