sslh-select: 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/sbin/sslh-select - from package: sslh

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

On Debian possible default full paths are:

/usr/sbin/sslh-select - from package: sslh

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

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