ssh-installkeys: 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/ssh-installkeys - from package: ssh-installkeys

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

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