unifont-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/unifont-viewer - from package: unifont-bin

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

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