ssh-askpass-fullscreen: 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-askpass-fullscreen - from package: ssh-askpass-fullscreen

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