plymouth: 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:

/bin/plymouth - from package: plymouth
/usr/bin/plymouth - from package: plymouth

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

On Debian possible default full paths are:

/bin/plymouth - from package: plymouth

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

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