mount.tmpfs: 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:

/sbin/mount.tmpfs - from package: util-linux-ng

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

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