OpenVAS not detecting Apache2.x Server ETag Header

The mentioned VT is only reporting if the web server is returning an ETag in the form of:

ETag: inode-size-mtime

in its header which discloses the inode of a requested file. You could verify this against your server with a call similar to:

curl --silent -I https://example.com 2>&1 | grep -i etag

to see if it actually returns such an inode or not.

As the related vulnerability:

https://nvd.nist.gov/vuln/detail/CVE-2003-1418

is quite old and already fixed since longer time (either directly in the OpenBSD source code or in default Apache configurations: https://httpd.apache.org/docs/current/mod/core.html#fileetag) you probably won’t see any reports of this VT these days.