unison-2.40: 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/unison-2.40 - from package: unison240-gtk

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

On Debian possible default full paths are:

/usr/bin/unison-2.40 - from package: unison

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

Error in other languages:
unison-2.40: bevel nie gevind nie
unison-2.40: comandă negăsită
unison-2.40: comando não encontrado
unison-2.40: commande introuvable
unison-2.40: command not found
unison-2.40: käsku ei ole
unison-2.40: không tìm thấy lệnh
unison-2.40: komanda nerasta
unison-2.40: Kommando nicht gefunden.
unison-2.40: kommandot finns inte
unison-2.40: komut yok
unison-2.40: nie znaleziono polecenia
unison-2.40: níor aimsíodh an t-ordú
unison-2.40: no se encontró la orden
unison-2.40: no s'ha trobat l'ordre
unison-2.40: opdracht niet gevonden
unison-2.40: parancs nem található
unison-2.40: perintah tidak ditemukan
unison-2.40: príkaz nenájdený
unison-2.40: příkaz nenalezen
unison-2.40: команда не найдена
unison-2.40: командата не е открита
unison-2.40: 命令找不到
unison-2.40: コマンドが見つかりません