Comment 2 for bug 1868625

Thank you for taking the time to report this issue and helping to make Ubuntu better.

Ubuntu 18.04 LTS has two tested & supported upgrade paths.
(1) Firstly to the next release, ie. Ubuntu 18.10. Closed now because 18.10 is EOL
(2) Secondly to the next LTS release, once 20.04.1 has been released; (likely August 2020) and isn't available yet.

The error message you obtained also says

"* Unofficial software packages not provided by Ubuntu
Please use the tool 'ppa-purge' from the ppa-purge
package to remove software from a Launchpad PPA and
try the upgrade again."

which is your reason it appears. I picked a package listed in the VarLogDistupgradeAptLog and explore on packages.ubuntu.com, and you have a package that isn't from a Ubuntu source (bluez from PPA; a 3rd party source). The error message I believe is exact, as you haven't removed 3rd party PPA & unofficial sources in your attempt to release-upgrade on your chosen untested & unsupported path.

I've marked this incomplete, as you haven't followed the instructions of the error message; or remove all 3rd party software before upgrade to proceed.

Also for best results I suggest sticking to tested & supported upgrade paths. If you believe I'm in error, you can return status to 'New' after leaving a message explaining why, or what I've missed.