The error is:
The Unix operating system (SLES 11 SP4) running on the remote host is no longer supported. Lack of support implies that no new security patches for the product will be released by the vendor. As a result, it is likely to contain security vulnerabilities.
All the SLES 11 SP4 patches that are necessary for running the OES services securely and with stability are made available via the OES 2015 SP1 update channels. So, even if SLES 11 SP4 is out of General Support, it does not impact the OES customers.
You can safely ignore the Security Vulnerability error given by the Nessus Scan report as it does not consider that SLES 11 SP4 is in Long Term Service Pack Support (LTSS) with the OES customers. We take security and stability of the OES servers seriously and we know they are a priority for our customers and hence are a priority for us too.
When you install OES 2015 SP1 using the add-on media, the Import Untrusted GnuPG Key pop-up is displayed. Import the key and then proceed.
Micro Focus plans to fix this issue in a future OES release.