On SecInfo check if CVE, CPEs and others are available or not, is none of them are available, probably the SCAP data base is missing, follow this link to solve it SCAP Issues. Is very important to ready the description and follow the step-by-step instructions to solve the issue.
It is clear that the installation is not complete reason why it is missing NVTs.
I suggest you to re-run the command sudo gvm-setup to see if it fix the issue.
Note: After the process is done, login to GSA and got to Feed status and if you see: updating in process, wait until the process if finished and go to NVTs to see if the problem persists or not.
I was able to get Ubuntu 20 and version 2.1 to work based on the clues posted in this thread. Basically I copied the Full and Fast scan configuration, then enabled the checkbox for Port scanners and chose Nmap (NASL Wrapper).