I think Parrot 5.0 is currently lacking Greenbone Vulnerability Management/OpenVAS. (I just installed Parrot Security 4.11.3 and am having apt issues upgrading to 5.0 via sudo apt install parrot-core.)
But when I was testing the Parrot 5.0 beta builds, OpenVAS (gvm) was not available on the servers.
I think I was able to add the Kali repository to sources.list (and the Kali gpg key) to install gvm, then commented out the repo.
Nevermind. I upgraded 4.11.3 to “5.0” and GVM is installed. Maybe the mirror you are using doesn’t have it? I’m using the following in my /etc/apt/sources.list.d/parrot.list
deb https://deb.parrot.sh/parrot parrot main contrib non-free