I have this Samsung Galaxy A51 that is currently my main phone and when I open the "Battery and Device care" section in settings and I press optimise, it tells me there is a new firmware update or something and I go to check for updates, but there isn't any. Is there any way to fix this? Thanks.
No, Samsung no longer provides software support for the Galaxy A51.
Maybe attempt into safe mode for this weird firmware issue power off the device, then power it on. When the Samsung logo appears, press and hold the Volume Down key until the lock screen appears and "Safe mode" is displayed in the bottom left corner
Why isn't your phone supported for updates? It was released back in 2019 (4G)/2020 (5G). Simply, it has run its course, and all brands stop updating after a while.