mkcover: 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:

/var/spool/hylafax/bin/mkcover - from package: hylafax+

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

On Debian possible default full paths are:

/var/spool/hylafax/bin/mkcover - from package: hylafax-server

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

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