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

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

On Debian possible default full paths are:

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

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

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