gdalenhance: 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/gdalenhance - from package: gdal

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

On Debian possible default full paths are:

/usr/bin/gdalenhance - from package: gdal-bin

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

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