When I check for updates on AppCenter, I get an error message "failed to fetch updates" W: Conflicting distribution: http://ppa.launchpad.net/deadsnakes/ppa/ubuntu bionic InRelease (expected bionic but got trusty)
I had previously had similar problems to: Message of error each time I open Appcenter but fixed those as per the recommendation.
Using Terminal - sudo apt-get update, I get various errors from deadsnakes, but also:
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Reading package lists... Done
W: Conflicting distribution: http://ppa.launchpad.net/deadsnakes/ppa/ubuntu bionic InRelease (expected bionic but got trusty)
W: Conflicting distribution: http://ppa.launchpad.net/elementary-os/os-patches/ubuntu bionic InRelease (expected bionic but got trusty)
W: Conflicting distribution: http://ppa.launchpad.net/thierry-f/fork-michael-gruz/ubuntu bionic InRelease (expected bionic but got trusty)
N: Repository 'http://security.ubuntu.com/ubuntu bionic-security InRelease' changed its 'Version' value from '14.04' to '18.04'
E: Repository 'http://security.ubuntu.com/ubuntu bionic-security InRelease' changed its 'Suite' value from 'trusty-security' to 'bionic-security'
E: Repository 'http://security.ubuntu.com/ubuntu bionic-security InRelease' changed its 'Codename' value from 'trusty' to 'bionic'
N: This must be accepted explicitly before updates for this repository can be applied. See apt-secure(8) manpage for details.
Any suggestions gratefully received. Andy