Hello GratefulAna, welcome to the Greenbone Community!
A full vulnerability scan of a Metasploitable 2 should indeed show a large number of vulnerabilities. I was not able to reproduce this problem on my GSM TRIAL 21.04.2 however, so we will have to investigate this step by step.
First, to check for some basic things:
Which scan config did you use? For a vulnerability scan, “Full and fast” should be used. The discovery configs just gather basic information about a system.
Which port list did you use? “All IANA assigned TCP” is recommended for starters. Other port lists may not include the ports required for some essential vulnerability tests.