For gvmd-20.08 and gvmd-21.04/master PostgreSQL 11 is our reference. The PostgreSQL 12 issue might get fixed but we can’t promise anything at the moment.
For other versions below gvmd-20.08 Debian Stretch is the base system and therefore PostgreSQL 9.6 is recommended.
For your current issue with the SCAP db, please try to use gvmd-9.0.1 the latest bugfix release instead of 9.0.0. I can remember either a ticket a GitHub or a topic in this forum where this issue was mentioned to and updating to the bugfix release fixed it.