uptime: 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/uptime - from package: procps

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

On Debian possible default full paths are:

/usr/bin/uptime - from package: procps

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

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