FBReader: 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/FBReader - from package: fbreader

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

On Debian possible default full paths are:

/usr/bin/FBReader - from package: fbreader

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

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