I actually wasn't able to get the web installer to work on an Ubuntu-based machine, had to do it on a Mac. Apparently it's a known issue with GNOME.
Discussion
That's surprising. Is there a Github issue already filed?
I'm not sure. I actually don't remember exactly what the issue was but they have this in the web install instructions:
"On Linux, GNOME has a bug causing compatibility issues with the installation process. It wrongly detects the phone in fastboot mode or fastbootd mode as being an MTP device and claims exclusive control over it. This will block the install process from proceeding."
There's a workaround but I either didn't try it or it didn't work.
I used it on Gnome, follow this guide to get it working - https://gist.github.com/smac89/251368c8df2ccd645baedc3e1238fdb4 Edit: I shared the wrong link before, sorry.