shellcheck: 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/shellcheck - from package: shellcheck

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

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