checkrestart: command not found

Have You tried accessing the binary with the full path?

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/sbin/checkrestart - from package: debian-goodies

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