mypaint: 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 Debian possible default full paths are:

/usr/bin/mypaint - from package: mypaint

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

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