crontab: 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/crontab - from package: cronie

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

On Debian possible default full paths are:

/usr/bin/crontab - from package: bcron-run
/usr/bin/crontab - from package: cron
/usr/bin/crontab - from package: systemd-cron

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

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