eric4-diff: 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/eric4-diff - from package: eric

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

Error in other languages:
eric4-diff: bevel nie gevind nie
eric4-diff: comandă negăsită
eric4-diff: comando não encontrado
eric4-diff: commande introuvable
eric4-diff: command not found
eric4-diff: käsku ei ole
eric4-diff: không tìm thấy lệnh
eric4-diff: komanda nerasta
eric4-diff: Kommando nicht gefunden.
eric4-diff: kommandot finns inte
eric4-diff: komut yok
eric4-diff: nie znaleziono polecenia
eric4-diff: níor aimsíodh an t-ordú
eric4-diff: no se encontró la orden
eric4-diff: no s'ha trobat l'ordre
eric4-diff: opdracht niet gevonden
eric4-diff: parancs nem található
eric4-diff: perintah tidak ditemukan
eric4-diff: príkaz nenájdený
eric4-diff: příkaz nenalezen
eric4-diff: команда не найдена
eric4-diff: командата не е открита
eric4-diff: 命令找不到
eric4-diff: コマンドが見つかりません