mosh-server: 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/mosh-server - from package: mosh

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

On Debian possible default full paths are:

/usr/bin/mosh-server - from package: mosh

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

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