Thanks @SuzieQ for your useful remarks on this “internal error” which seems to have no effect luckily. I also added a comment to @breversa message about having the same warning on installing on a Galaxy A5.
It’s curious that these older Samsung devices have the same problem if, as you say, the Lineage version does not. Since /e/ is based on Lineage, one wonders how it happens, if they have a script that gets rid of the warning since my guess is that it is built into the original AOSP to warn users of possible bugs when the upgrade is made from an earlier version. Why would they say “contact your manufacturer”? Lineage doesn’t make cellphones as far as I know, so it must be aimed at all manufacturers to tell them to upgrade to a more recent Android. Google putting pressure on builders as they are doing with the move to A/B devices. Samsung is in the firing line!