Is this your correct CUSA code? You have the US version of the game? If it's saying "unknown" then I'm guessing either you are US but don't have the PS4 version installed AND are offline, or this just isn't your region.
If you are definitely US and this is definitely your CUSA code (if you're unsure then there's a troubleshooting FAQ that explains how to check) then while I don't have the game, if you're loading the PS5 version of the game, I can think of a couple of possible issues I've seen with other games (again not this one since I don't have it yet).
Reason one, sometimes it matters if the PS4 game and PS5 game are in the same place. Meaning, the PS5 game not see your PS4 save if it doesn't see the PS4 game, so if you don't have the PS4 version currently downloaded/installed or if it's downloaded installed on an external USB (even though it might be plugged in!) the PS5 game might not see it and so assume there are no PS4 saves to read. I think I've only seen this once, but just in case download the PS4 version if it's not downloaded, and stick it on console storage where the PS5 version is.
Reason two, the game might only be set up to read cloud saves. The way you say the save says "unknown" makes me think you're launching the game offline (otherwise your PS5 would read the metadata and figure out from the PSN store what the game is, even if it is the wrong region, and tell you what game it is, as long as you've resigned it correctly). So launch the PS5 game online and see if you see the saves that way. If they're still not there, then you might have to load the PS4 version first and let that version sync the saves to some cloud system the game uses.
Would be easier to give you definitive answers if I had time to keep up with new releases!