Vulnerability not detected

Hi All,

I am new to GCE. I wanted to evaluate my system for vulnerabilities and tried with GCE GOS Version 6.0.7. The report did not show any errors. But when I tried to evaluate with openVAS it reported the following error.
“Medium (CVSS: 4.8) NVT: FTP Unencrypted Cleartext Login”
Can someone help me to understand the reason.