After a brand new install, I updated elementary (5.1.7) and wingpanel disappeared on the top.
On reboot I get the "starting the session has failed." error when I try to login.
Realizing I could just install wingpanel (after looking into other people with similar problems) I found a bigger issue - the repository's wingpanel requires libplank1, which requires an exact version of libplank-common which is not on the repo. Specifically it wants libplank-common (= 0.11.89+r1647+pkg5~ubuntu220.127.116.11) but the repo has 18.104.22.168 I have tried all the other fixes short of a nuke and pave of elementary and not a single one has worked.