hg: 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/hg - from package: mercurial

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

On Debian possible default full paths are:

/usr/bin/hg - from package: mercurial

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

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