I try to update from El Capitan to Big Sur, but get a message saying Big Sur can't be loaded.
How do I upgrade from El Capitan to Big Sur?
iMac, OS X 10.11
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Is your Mac even compatible with Big Sur and does the Mac meet the minimum system requirements? See this article to make sure Big Sur is compatible with your Mac:
macOS Big Sur - Technical Specifications
https://eshop.macsales.com/guides/Mac_OS_X_Compatibility
If your Mac is compatible with Big Sur and you have at least 36GB of free storage space and you get an error, then perhaps you have some third party software installed which is interfering. You can try booting into Safe Mode and trying to run the Big Sur installer app again. Anti-virus apps, cleaning apps, and third party security software are not needed on a Mac and can interfere with upgrading macOS. Uninstalling these apps by following the developer's instructions is recommended for better performance of your Mac.
If you still have issues, then try creating and using a bootable macOS Big Sur USB installer using the instructions in this Apple article:
https://support.apple.com/en-us/HT201372
When using a bootable macOS Big Sur USB installer you will need at least 45GB of free storage space on Mac's drive. If you still have problems, then you may want to first try installing macOS 10.13 High Sierra, before attempting to install Big Sur again.
Before attempting to upgrade macOS make sure to have a good backup just in case something goes wrong. You should always have frequent and regular backups if you value your data.