I did a clean installation of opensuse 15.2 i updated the system and added
Packman repo. Later i also added by terminal flathub repo with the following
command: 
flatpak remote-add --if-not-exists flathub
https://flathub.org/repo/flathub.flatpakrepo but the system responded to me
this way:
error: While opening repository /var/lib/flatpak/repo: opendir(objects): No
such file or directory
Searching on google I applied the "solution" that was recommended on reddit,
basically make: 
sudo rm -r /var/lib/flatpak/repo and then run the command again to add the
flathub repo. This procedure was actually successful and I was able to install
some flatpak applications e.g.: spotify and it works correctly.

thank you for your attention