bpython3: 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/bpython3 - from package: bpython3

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

Error in other languages:
bpython3: bevel nie gevind nie
bpython3: comandă negăsită
bpython3: comando não encontrado
bpython3: commande introuvable
bpython3: command not found
bpython3: käsku ei ole
bpython3: không tìm thấy lệnh
bpython3: komanda nerasta
bpython3: Kommando nicht gefunden.
bpython3: kommandot finns inte
bpython3: komut yok
bpython3: nie znaleziono polecenia
bpython3: níor aimsíodh an t-ordú
bpython3: no se encontró la orden
bpython3: no s'ha trobat l'ordre
bpython3: opdracht niet gevonden
bpython3: parancs nem található
bpython3: perintah tidak ditemukan
bpython3: príkaz nenájdený
bpython3: příkaz nenalezen
bpython3: команда не найдена
bpython3: командата не е открита
bpython3: 命令找不到
bpython3: コマンドが見つかりません