scan: 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/lib64/mpich/bin/scan - from package: scalasca-mpich
/usr/lib64/openmpi/bin/scan - from package: scalasca-openmpi

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

On Debian possible default full paths are:

/usr/bin/scan - from package: dvb-apps
/usr/bin/mu-mh/scan - from package: mailutils-mh
/usr/bin/mh/scan - from package: nmh

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

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