expression_levels.pl: 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/share/gbrowse/htdocs/tutorial/bin/expression_levels.pl - from package: gbrowse

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

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