opj_viewer: 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 Debian possible default full paths are:

/usr/bin/opj_viewer - from package: openjpip-viewer

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

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