waitress-serve-python3: 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/waitress-serve-python3 - from package: python3-waitress

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

Error in other languages:
waitress-serve-python3: bevel nie gevind nie
waitress-serve-python3: comandă negăsită
waitress-serve-python3: comando não encontrado
waitress-serve-python3: commande introuvable
waitress-serve-python3: command not found
waitress-serve-python3: käsku ei ole
waitress-serve-python3: không tìm thấy lệnh
waitress-serve-python3: komanda nerasta
waitress-serve-python3: Kommando nicht gefunden.
waitress-serve-python3: kommandot finns inte
waitress-serve-python3: komut yok
waitress-serve-python3: nie znaleziono polecenia
waitress-serve-python3: níor aimsíodh an t-ordú
waitress-serve-python3: no se encontró la orden
waitress-serve-python3: no s'ha trobat l'ordre
waitress-serve-python3: opdracht niet gevonden
waitress-serve-python3: parancs nem található
waitress-serve-python3: perintah tidak ditemukan
waitress-serve-python3: príkaz nenájdený
waitress-serve-python3: příkaz nenalezen
waitress-serve-python3: команда не найдена
waitress-serve-python3: командата не е открита
waitress-serve-python3: 命令找不到
waitress-serve-python3: コマンドが見つかりません