Sorry I thought that you were telling about project compilation so I said about the project log location. unreal engine build log does not get written to file sometimes though it shows in terminal that it is being logged to file.
I did a fresh install of ubuntu 20.04 LTS (will be same as your kubuntu version). Then did apt update, apt upgrade, then build-essential without any other things. Was able to compile engine without error. I also set nvidia drivers to 470.x (it is not relevent for building the engine, but ue4.26.2 and above will not run well without this driver, so set this in additional drivers)
Try these and see
delete the clang folders under
UnrealEngine/Engine/Extras/ThirdPartyNotUE/SDKs/HostLinux
also delete all clang-**-Centos*.tar.gz (if you know which sdk is used for your UE version then that specific one)
UnrealEngine/.git/ue4-sdks/
#then again run from konsole with internet connected
./Setup.sh
./GenerateProjectFiles.sh
make sure there were no errors in the above commands
#Then clean the CrashReportClient by
make ARGS=“-clean” CrashReportClient
#then compile only the crash CrashReportClient
make ARGS=“-verbose” CrashReportClient
#if there were no errors then make the full set of required tools by as usual
make