I am not an expert here, but one of my colleagues says to widen your search criteria.
It looks like you are narrowly looking for a major.minor.patch.hotfix (9.16.4.18) and you should widen the scope to just major versions like 9... so the call may look like something like this: GET "https://apix.cisco.com/security/advisories/v2/OSType/asa?version=9"
It will return anything that starts with 9.x.x.x. Then you can look at the replies to make sure you are formatting the version in the manner the API is expecting it.
On another product, you can see a 'w' in the version https://developer.cisco.com/docs/psirt/#!obtain-advisory-by-software/3-retrieve-all-advisories-impacting-a-specific-version-of-cisco-network-operating-system
If you search without the 'w', it would fail. It has to be exactly like the API wants it. And you'll know how the API wants it by doing a wider search, look at the reply, and see how the API formatted it.
