System Settings can't be built (and therefore Plasma can't be built, at least using kde-builder). This error message is seen: "ninja: error: '/home/user/kde/src/systemsettings/po/sl/docs/systemsettings/index.docbook', needed by 'po/sl/docs/systemsettings/index.cache.bz2', missing and no known rule to make it". That error happens since that commit was done: https://invent.kde.org/plasma/systemsettings/-/commit/c8856e71aeee805630bd3ed2b7e5bb9451d4e04d which erased the "po/sl/docs/systemsettings/index.docbook" file. Thanks for all the efforts.
Please use the matrix channels (https://community.kde.org/Matrix) or the forum (https://discuss.kde.org/) for support on compiling KDE software. In this particular case, refreshing the build system might fix things. (With kdesrc-build/kde-builder, pass --refresh-build flag)