Why can't I update my 2013 iMac to Big Sur?
I have a 2013 iMac that works just fine for my needs, but I need to install new software that requires Big Sur 11.0, and I have been unable to update to Big Sur. Why is this?
iMac 21.5″, macOS 10.15
macOS Catalina is the newest compatible upgrade. Big Sur is not possible but Apple doesn't give the reason why. See the following from Identify your iMac model - Apple Support
iMac (21.5-inch, Late 2013)
Model Identifier: iMac14,1
Part Number: ME086xx/A, ME087xx/A
Newest compatible operating system: macOS Catalina