RNAhybrid: 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/RNAhybrid - from package: rnahybrid

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

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