ispell: command not found

Have You tried accessing the binary with the full path?

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/ispell - from package: aspell

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

On Debian possible default full paths are:

/usr/bin/ispell - from package: ispell
/usr/bin/ispell - from package: tmispell-voikko

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