nvimgdiff: 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/nvimgdiff - from package: libnvtt-bin

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

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