certmgr: 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/certmgr - from package: mono-devel

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

On Debian possible default full paths are:

/usr/bin/certmgr - from package: mono-devel

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

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