Hi Steve,
From the wording of the reported vulnerability —
Web Application Potentially Vulnerable to Clickjacking — I'd hazard a guess your scanning tool is
Nessus. We use Nessus but it didn't report Vulnerable to Clickjacking against our AtoM 2.6.4 VMs. However, this is likely because some years ago I made customisations to the Nginx configuration specifically to prevent clickjacking, rather than because we're on the latest AtoM release.
I think what you need to do is add the following to your Nginx configuration, inside the main server { } block:
add_header X-Frame-Options DENY;
add_header Content-Security-Policy "frame-ancestors 'none'";
This assumes you don't need to allow other sites to frame/iframe your AtoM site. We do, so we have a slightly different configuration. I believe you need both lines because of varying levels of browser support for the two headers. You'll need to run systemctl reload nginx for the configuration change to take effect. Then run another scan and see if the vulnerability is still reported. You'll also want to test the site to make sure the change hasn't had any unintended consequences.
See also
this web page as a reference to how to protect against clickjacking.
In our case Nessus did pick up on other things, but my conclusion was that they were all false positives. I'm happy to share details off-list (and/or via the security email address, Dan) if this would be helpful.
Thanks, Jim