mysql_secure_installation: 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/mysql_secure_installation - from package: mysql55w-server
/usr/bin/mysql_secure_installation - from package: mysql-server

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

On Debian possible default full paths are:

/usr/bin/mysql_secure_installation - from package: mariadb-server-10.0
/usr/bin/mysql_secure_installation - from package: mysql-server-5.5

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

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