cython3: 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/cython3 - from package: cython3

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

Error in other languages:
cython3: bevel nie gevind nie
cython3: comandă negăsită
cython3: comando não encontrado
cython3: commande introuvable
cython3: command not found
cython3: käsku ei ole
cython3: không tìm thấy lệnh
cython3: komanda nerasta
cython3: Kommando nicht gefunden.
cython3: kommandot finns inte
cython3: komut yok
cython3: nie znaleziono polecenia
cython3: níor aimsíodh an t-ordú
cython3: no se encontró la orden
cython3: no s'ha trobat l'ordre
cython3: opdracht niet gevonden
cython3: parancs nem található
cython3: perintah tidak ditemukan
cython3: príkaz nenájdený
cython3: příkaz nenalezen
cython3: команда не найдена
cython3: командата не е открита
cython3: 命令找不到
cython3: コマンドが見つかりません