Sorry, but can you explain what you mean? Except for whatever printed documents came with your shipped software, I’ve never heard of a “software serial number” for AS/400 Licensed Program Products. If you didn’t keep the paperwork, then I don’t think there’s any record stored in your system. Call IBM.
You might be able to log into your IBM account through the web and find something about “software serial number” there, but I don’t recall seeing anything similar under my accounts.
If you describe what you’re looking for, maybe it’ll ring a bell about a different term for it.