curvecpclient: 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 CentOS possible default full paths are:

/usr/bin/curvecpclient - from package: nacl

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/curvecpclient - from package: nacl-tools

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

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