Big Sur Installation

Welcome to Apple Support Communities.

Sorry, but your iMac is not compatible with macOS Big Sur 11. The latest/last supported version of macOS for your Mac is macOS Catalina 10.15.

-Jack

Please start a new thread on Apple Support Communities and post an EtreCheck report following the instructions below:

1. Navigate to: https://etrecheck.com and get the free version of EtreCheckPro.
2. Download it. It is a free and trusted app. It is often recommended here on Apple Support Communities. 
3. Open EtreCheck, and before running the report, be sure to check “Allow full Drive Access” at the bottom of the EtreCheckPro window.
4. Run the report, and put it in a post, as seen here: https://discussions.apple.com/docs/DOC-250000211. We will then examine the report for issues in your Mac.

-Jack

After upgrading to Big Sur, my Mac seems to be running VERY slow. Is there an issue with this software version? I have an iMac Pro 21.5" Late 2015.

Is there a way to go back a version?

Jerry