trac-admin: 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/trac-admin - from package: trac10
/usr/bin/trac-admin - from package: trac

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

On Debian possible default full paths are:

/usr/bin/trac-admin - from package: trac

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

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