sha256sum: 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 CentOS possible default full paths are:

/usr/bin/sha256sum - from package: coreutils

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/sha256sum - from package: coreutils

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

Error in other languages:
sha256sum: bevel nie gevind nie
sha256sum: comandă negăsită
sha256sum: comando não encontrado
sha256sum: commande introuvable
sha256sum: command not found
sha256sum: käsku ei ole
sha256sum: không tìm thấy lệnh
sha256sum: komanda nerasta
sha256sum: Kommando nicht gefunden.
sha256sum: kommandot finns inte
sha256sum: komut yok
sha256sum: nie znaleziono polecenia
sha256sum: níor aimsíodh an t-ordú
sha256sum: no se encontró la orden
sha256sum: no s'ha trobat l'ordre
sha256sum: opdracht niet gevonden
sha256sum: parancs nem található
sha256sum: perintah tidak ditemukan
sha256sum: príkaz nenájdený
sha256sum: příkaz nenalezen
sha256sum: команда не найдена
sha256sum: командата не е открита
sha256sum: 命令找不到
sha256sum: コマンドが見つかりません