modula3_count: 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/modula3_count - from package: sloccount

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

On Debian possible default full paths are:

/usr/bin/modula3_count - from package: sloccount

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

Error in other languages:
modula3_count: bevel nie gevind nie
modula3_count: comandă negăsită
modula3_count: comando não encontrado
modula3_count: commande introuvable
modula3_count: command not found
modula3_count: käsku ei ole
modula3_count: không tìm thấy lệnh
modula3_count: komanda nerasta
modula3_count: Kommando nicht gefunden.
modula3_count: kommandot finns inte
modula3_count: komut yok
modula3_count: nie znaleziono polecenia
modula3_count: níor aimsíodh an t-ordú
modula3_count: no se encontró la orden
modula3_count: no s'ha trobat l'ordre
modula3_count: opdracht niet gevonden
modula3_count: parancs nem található
modula3_count: perintah tidak ditemukan
modula3_count: príkaz nenájdený
modula3_count: příkaz nenalezen
modula3_count: команда не найдена
modula3_count: командата не е открита
modula3_count: 命令找不到
modula3_count: コマンドが見つかりません