plymouth-log-viewer: 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/plymouth-log-viewer - from package: plymouth-utils

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

On Debian possible default full paths are:

/usr/bin/plymouth-log-viewer - from package: plymouth-x11

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

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