Mac OS Won't Update
I'm using a MacBook Air mid-2012, and I'm trying to update the OS, but I am having difficulty. The MacBook is currently running Catalina 10.15.7, and the only update I have available is "MacOS Catalina Security Update 2021-004 10.15.7". Each time I try to update, the system asks me to restart and then download. However, I seem to be in a loop of restarting, trying to download and then the system asking me to restart before the update each time.
I have also tried to download Big Sur, which is "available on this Mac", - as the app store tells me, but there is an error if I try to download it.
Any help would be much appreciated.
MacBook Air 13″, macOS 10.15