is.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/ledgersmb/bin/is.pl - from package: ledgersmb
/usr/share/sql-ledger/bin/lynx/is.pl - from package: sql-ledger
/usr/share/sql-ledger/bin/mozilla/is.pl - from package: sql-ledger

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

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