speedtest-cli: command not found

Have You tried accessing the binary with the full path?

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH

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/speedtest-cli - from package: speedtest-cli

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