Wayland is the default session for KaOS since February. Presuming you are using this default, then Smplayer can't work.
See the announcement, and why Smplayer is no longer default:
https://kaosx.us/news/2022/kaos02/
Haruna is the replacement. Or you can still use an X session.
If you don't use autologin, you can select X from the dropdown menu on logging in, if you do use autologin, you can adjust the selected session with Systemsettings > Startup & Shutdown > Login Screen (SDDM) > Behavior and switch with session
to Plasma (X11).