Thanks a lot Brian it worked. removed kubuntu-backports and oibaf/graphics-drivers ppa using the ppa-purge then update issue was fixed and now upgraded to 18.04. Thanks again for your valuable time.
Thanks a lot Brian it worked. removed kubuntu-backports and oibaf/graphics- drivers ppa using the ppa-purge then update issue was fixed and now upgraded to 18.04. Thanks again for your valuable time.