KDE Linux/Use KeepassXC with Web Browsers
Because web browsers in KDE Linux are sandboxed Flatpaks, there are some limitations in how they can interact with other apps that are very difficult to work around. Unfortunately, this includes KeepassXC.
At the moment, no simple solution exists, and general users are recommended to choose a different method of managing passwords.
Advanced users comfortable with tinkering and recovering from breakage, cam try out the highly experimental steps described at https://www.snamellit.com/blog/keepassxc-and-flatpak or https://discourse.flathub.org/t/how-to-run-firefox-and-keepassxc-in-a-flatpak-and-get-the-keepassxc-browser-add-on-to-work/437.