In app centre it is showing failed to fetch updates

Question

E: The repository 'http://ppa.launchpad.net/webupd8team/tor-browser/ubuntu bionic Release' does not have a Release file.
W: Updating from such a repository can't be done securely, and is therefore disabled by default.
W: See apt-secure(8) manpage for repository creation and user configuration details

Answer 1

The tor-browser package in that ppa has not been updated in a very long time, I do not recommend using it. My suggestion is to remove that ppa, to get AppCenter working again, and then install tor from the ubuntu repository (as per the Tor Project's instructions).

To remove the obsolete ppa:

sudo add-apt-repository --remove ppa:webupd8team/tor-browser

After that, run sudo apt update to see if the error is gone. If all is well, try running sudo apt upgrade to bring your system up-to-date.