On PS5, the only way to backup your game save files is through the PlayStation Plus subscription service.
If you're trying to backup your save files on your PS5 for next-gen games, you're going to need a PS Plus subscription. Whereas you could have copied your game save files over to a USB stick on PS4, there's no such option for PS5 game save files, so you'll need to back them up to the cloud, which is locked behind the PS Plus subscription.
This effectively means that backing up your game save files on Sony's next-gen console is locked behind a paywall. Note however that you can still back up game save files from your PS4 on your PS5 to a USB stick. So if you were playing Call of Duty: Black Ops Cold War for example on PS4 and transfer your save data over to PS5, you could then back up that save file to a USB stick. But if you started playing Assassin's Creed Valhalla on PS5, you couldn't transfer that save to a USB stick.
There's no information to go on as to why Sony has changed this policy for its next-gen console. Backing up your save data on PS4 to a memory stick was painless and simple, and changing this poses a problem for PS5 players who don't have a PS Plus subscription.
Being a PS Plus subscriber has additional benefits, of course. You'll get access to a list of PS Plus Collection games on PS5, including the likes of God of War, Monster Hunter: World, and Persona 5, as well as getting two free games per month, like Bugsnax.
For a list of other games like Assassin's Creed Valhalla, that can be upgraded to run on Sony's next-gen console for free, head over to our guide on PS5 upgrade games for more. And if you haven't got your machine yet, here's where to check on the PS5 price.