Pamac isn't able to install flatpak and pacman packages together.
Pamac 9.5.6-1 on ArchLinux
When i choose to install flatpak packages, it is working, it installs them. When i choose pacman ones, it works, but when i mix flatpak and pacman packages selection it will show me an error, that package in my case org.libreoffice.LibreOffice is not existing. I think it is because it tries to install flatpaks using pacman.