Same issue on my Pixel 4a. Web Installer Unpacks and writes bootloader, then Errors out with "Error: Unable to claim interface."
Pixel 6 Pro - Failed to execute ... Unable to claim interface ...
clicking on the "unlock bootloader" button again seems to reset something, as the process takes longer, installs radio and some other things, but ultimately fails with the same error in the web installer, but leaving me in fastbootd instead of the bootloader screen.
Thought I'd try to revert to google's stock OS, but after clicking on the remove-non-stock key button in the web installer, I get this error:
Error: Bootloader replied with FAIL: Partition doesn't exist
So what? Did the web installer brick me?
OK -- so I think I found the problem.
Earlier, I had flashed a Pixel 4 -- and I still had the web installer tab open from that install.
Closing that tab allowed me to successfully flash the 4a without trouble from the 'unable to claim interface' error.
My best guess is that the "interface couldn't be claimed" because the 2 tabs were jousting for supremacy.