knotify4: 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/knotify4 - from package: kdebase-runtime

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

On Debian possible default full paths are:

/usr/bin/knotify4 - from package: kde-runtime

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

Error in other languages:
knotify4: bevel nie gevind nie
knotify4: comandă negăsită
knotify4: comando não encontrado
knotify4: commande introuvable
knotify4: command not found
knotify4: käsku ei ole
knotify4: không tìm thấy lệnh
knotify4: komanda nerasta
knotify4: Kommando nicht gefunden.
knotify4: kommandot finns inte
knotify4: komut yok
knotify4: nie znaleziono polecenia
knotify4: níor aimsíodh an t-ordú
knotify4: no se encontró la orden
knotify4: no s'ha trobat l'ordre
knotify4: opdracht niet gevonden
knotify4: parancs nem található
knotify4: perintah tidak ditemukan
knotify4: príkaz nenájdený
knotify4: příkaz nenalezen
knotify4: команда не найдена
knotify4: командата не е открита
knotify4: 命令找不到
knotify4: コマンドが見つかりません