sss_ssh_knownhostsproxy: 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/sss_ssh_knownhostsproxy - from package: sssd-common

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

On Debian possible default full paths are:

/usr/bin/sss_ssh_knownhostsproxy - from package: sssd-common

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

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