hg-ssh: 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-ssh - from package: mercurial

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

On Debian possible default full paths are:

/usr/bin/hg-ssh - from package: mercurial-common

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

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