leonconv: 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/lib/x86_64-linux-gnu/gap/pkg/guava/bin/leonconv - from package: gap-guava-bin

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

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