I found that if upon receiving this error, before booting into LineageOS I can
Boot into the bootloader from TWRP,
Click on Reboot
Click on Bootloader
Boot into a new TWRP from the bootloader
Run twrp boot ./twrp.*.img
I can then install the gapps.
What cause this? Well my theory now is that the instructions on the Lineage OS suggest Wiping the System ...
addonsu is an open source rooting solution from LineageOS. Magisk is another rooting solution. So are/were SuperSU, PHH's SuperUser, CF-Auto-Root and so on. You cannot / don't need to / should not use multiple rooting solutions simultaneously. Magisk - during flashing process - deletes any files related to other rooting solutions, including /system/xbin/su ...
Typical messaging apps like Rocket.Chat use Firebase Cloud Messaging (FCM formerly known as GCM) for retrieving notifications of new messages from the server.
An Android side FCM required the Google Play Services app.
However LineageOS does not include any Google apps by default. Therefore all apps that use push notifications via Google/FCM will not work ...