turnutils_uclient: 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/turnutils_uclient - from package: coturn
/usr/bin/turnutils_uclient - from package: rfc5766-turn-server

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

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