plymouth-set-default-theme: 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/sbin/plymouth-set-default-theme - from package: plymouth-scripts

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

On Debian possible default full paths are:

/usr/sbin/plymouth-set-default-theme - from package: plymouth

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

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