Can't enter recovery Mode on Imac 27 late 2015 Big Sur

Hi Geyzes,

For deleting all your data and re-installing macOS Big Sur, try these steps:

1. Sign out of your Apple ID in System Preferences —> Apple ID —> Overview —> Sign Out.
2. Shut down the Mac.
3. Turn on the Mac, and immediately press and hold Option + Command + R until you see a spinning globe.
4. Once you are in macOS Recovery, verify you are connected to WiFi in the top right corner.
5. Go to Disk Utility.
6. Select "View" in the toolbar —> "Show All Devices".
7. Select your main hard drive or SSD. This will either be the top, non indented hard drive. 
8. Click Erase, and input the following exactly: (If you cannot select erase on the main hard drive, erase the "Container" or “Macintosh HD”.)
9. Name: Macintosh HD
10. Format: APFS
11. Scheme: GUID Partition Map (if shown)
12. Click "Erase". If “Erase Volume Group” is shown, use that instead. 
13. Quit Disk Utility
14. Go to the reinstall macOS app.
15. Follow the prompts, and install on Macintosh HD.

Cheers,

Jack

command-R only works to restore Big Sur, if it was installed directly onto the computer and the installer created its own recovery partition, and the drive itself is healthy. The fact that command-R doesn't work, consider how it was first installed, and if it was installed that way, then it is possible the drive itself is failing.

Note, all recovery keys must be pressed with a WIRED keyboard. Command key on a non-Apple keyboard is the same as the Windows logo key, and Alt is the same as Option key.

you missed one:

command-option-SHIFT-R.

Have you tried Option + Command + R?

This downloads a brand new macOS Recovery system.

Glad it worked!