I was able to resolve this error in 20.08 by simply running
sudo greenbone-feed-sync --type GVMD_DATA
I believe this line is supposed to happen sometime during the gvm-setup script but it seems to fail sometimes.
I’ve seen recommendations to run gvm-setup more than once, so it makes sense that this particular line might not always work. Anyway, this command brought in the scan config information which resolved the issue.
[Update] Aha, just found this, which confirms the fact that the feeds bring in scan configs: