@gmiserbia1 This sounds like a known problem that we have since fixed, but you may need a one-time manual step to get unstuck…
If you are already on Endless OS 3.3 or later:
If you are on an earlier version of Endless OS and cannot update the operating system:
chmod +x eos-fix-ostree-repo
Enter your password when requested, and wait until the script finishes running before doing anything else. (In particular, please don’t open the app center while this operation is in progress.) Once the script is done, please try to upgrade again, and let us know if you still have any problems.
See discussion on https://community.endlessos.com/t/unable-to-update-install-anything/3794