ssh-import-id-gh: command not found

Have You tried accessing the binary with the full path?

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH

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 Debian possible default full paths are:

/usr/bin/ssh-import-id-gh - from package: ssh-import-id

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