As indicated by the message in the error message the problem (probably) lies with the first line in the file
/etc/apt/sources.list.d/raspi.list. The files under
sources.list.d are intended for conveniently adding extra repositories to APT and are added to the ones listed in
sources.list. You can think of them as "include" files for APT.
Check this particular file with the text editor, you'll need
sudo but seeing as you've already been able to check and edit your
sources.list I'll assume you know how to do this. The first line should be something along the lines of (based on what's on my Pi at least)
deb http://archive.raspberrypi.org/debian/ stretch main ui
Edit the line as required and close the editor and then try
sudo apt-get update.
The Raspbian pages are a little outdated so I'm not sure, short of burning the latest image to an SD card and having a look, where to check on what the latest & greatest contents of the file is supposed to be but this should get you up and running.