clamscan: 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/clamscan - from package: clamav

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

On Debian possible default full paths are:

/usr/bin/clamscan - from package: clamav

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

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