expchk: 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/expchk - from package: dnssec-tools

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

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