nyancat: 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
/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 Debian possible default full paths are:

/usr/bin/nyancat - from package: nyancat

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