I installed again Ubuntu 18.04 now. After, i tried update and upgrade commands. Upgrade command executed very well. But, update command didn't executed and gave error output. You can view error under:
sudo apt-get update
Hit:1 http://security.ubuntu.com/ubuntu bionic-security InRelease
Err:2 http://tr.archive.ubuntu.com/ubuntu bionic InRelease
Could not resolve 'tr.archive.ubuntu.com'
Err:3 http://tr.archive.ubuntu.com/ubuntu bionic-updates InRelease
Could not resolve 'tr.archive.ubuntu.com'
Err:4 http://tr.archive.ubuntu.com/ubuntu bionic-backports InRelease
Could not resolve 'tr.archive.ubuntu.com'
Reading package lists... Done
W: Failed to fetch http://tr.archive.ubuntu.com/ubuntu/dists/bionic/InRelease Could not resolve 'tr.archive.ubuntu.com'
W: Failed to fetch http://tr.archive.ubuntu.com/ubuntu/dists/bionic-updates/InRelease Could not resolve 'tr.archive.ubuntu.com'
W: Failed to fetch http://tr.archive.ubuntu.com/ubuntu/dists/bionic-backports/InRelease Could not resolve 'tr.archive.ubuntu.com'
W: Some index files failed to download. They have been ignored, or old ones used instead.