tkdiff: 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/tkdiff - from package: tkcvs

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

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