Thunderbolt Display Firmware Update Big Sure could not complete

1. Check Compatibility: Make sure that the Thunderbolt Display firmware update is compatible with your specific display model and macOS version. Check the official Apple support page for the Thunderbolt Display firmware update to ensure you have the correct version.
2. Restart and Retry: Sometimes, a simple restart of your Mac can resolve temporary glitches. After restarting, attempt the firmware update again.
3. Check Connection: Ensure that the Thunderbolt Display is properly connected to your Mac. Make sure the Thunderbolt cable is securely plugged into both the display and the Mac.
4. Reset NVRAM/PRAM: Resetting the NVRAM (non-volatile random-access memory) or PRAM (parameter random-access memory) on your Mac can sometimes resolve firmware-related issues. To reset NVRAM/PRAM, shut down your Mac, then turn it on and immediately press and hold Option + Command + P + R until you hear the startup sound a second time.
5. Update macOS: Make sure your macOS is fully up to date. Sometimes, firmware updates require the latest version of macOS to be installed. Go to Apple menu > System Preferences > Software Update to check for and install any available updates.
6. Safe Mode: Boot your Mac in Safe Mode by restarting and holding down the Shift key until the Apple logo appears. Safe Mode can help troubleshoot and isolate software-related issues that might be interfering with the firmware update.
7. Try Another Port: If your Mac has multiple Thunderbolt ports, try connecting the Thunderbolt Display to a different port and attempt the firmware update again.
8. Use Another Mac: If you have access to another compatible Mac, you can try performing the firmware update from that Mac to see if the issue is specific to your Mac.
9. Contact Apple Support: If none of the above steps work, it's advisable to contact Apple Support for further assistance. They can provide guidance specific to your situation and help troubleshoot the issue remotely.
10. Apple Store or Authorized Service Provider: If you're unable to resolve the issue on your own, you can visit an Apple Store or an Authorized Service Provider. They can diagnose and address hardware or software-related problems with your Thunderbolt Display and Mac.

Start here:

Genius Bar Reservation and Apple Support Options - Apple

Unfortunately, the user above is just copying and pasting things from something like ChatGPT. Hopefully, a real person will stop by. I wish I knew more about the matter.