ybmtopbm: 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/ybmtopbm - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/ybmtopbm - from package: netpbm

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

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