Welcome to NuTyX forums
I think you install NuTyX from base. You need then to install make-ca
sudo cards install make-ca
Flatpak is then running out of the box. The command:
flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo...
is not necessary on NuTyX
Best regards,
Thierry