I had the same issue with the TbT Retimer in a Dell XPS.  Each Thunderbolt (USB-C) port has a Retimer to boost signals.  If anything other than a power adapter is plugged in to one of these ports, it prevents the updating of the Retimer firmware.  A docking station will also prevent the updating.  I had an external screen plugged in which also delivered power, so I removed it and used a power adapter, then I had no more problems.
Hello,One of the first things I did after opening my laptop was run the Dell Command Update program to check if I have any updates, sure enough there were a few, including a critical BIOS one. I ran the updates and the thing that failed during flashing was the Intel Tbt retimer. I ran the update twice, same problem. Even tried to update after downloading the BIOS update online and the laptop just froze for half an hour. I had to shut it down from the button. When I go into BIOS Event Log I have two logs that say Alert! Firmware update is failed. At that point I decided to give up and wait for the next update. My main issue now is that the battery gets drained randomly, even when I shut down the laptop completely. It used to happen even when it was plugged in but changing the battery settings from adaptive to primarily AC use seems to have fixed that. Now I only get the issue when the laptop is unplugged. Another weird issue is that I don't see the Dell DA20 Adapter in the Thunderbolt Control Center. Also if I want to run the Dell Command Update now it just crashes and I have to repair it, then it starts, says completing update. After that it says update completed but if I want to run it again, same issue, crash->repair->updating->update complete. I also tried reinstalling some drivers from the device manager but without any success. Can you let me know what I can do to fix the battery issue at least ? Thanks!
XPS 15 9510 BIOS Update failed is causing issues