tkcon: 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/tkcon - from package: tkcon

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

On Debian possible default full paths are:

/usr/bin/tkcon - from package: tkcon

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

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