jarsigner: 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/lib/jvm/java-1.5.0-gcj-4.8-amd64/bin/jarsigner - from package: gcj-4.8-jdk
/usr/lib/jvm/java-1.5.0-gcj-4.9-amd64/bin/jarsigner - from package: gcj-4.9-jdk
/usr/lib/debug/usr/lib/jvm/java-7-openjdk-amd64/bin/jarsigner - from package: openjdk-7-dbg
/usr/lib/jvm/java-7-openjdk-amd64/bin/jarsigner - from package: openjdk-7-jdk

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

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