For now, on the sources.list:
sudo nano /etc/apt/sources.list
Set a # in front of deb http://raspbian.raspberrypi.org/raspbian/ buster main contrib non-free rpi
And make this new line:
The end file will be like this:
Code
# deb http://raspbian.raspberrypi.org/raspbian/ buster main contrib non-free rpi
deb http://ftp.cica.es/mirrors/Linux/raspbian/raspbian/ buster main contrib non-free rpi
Then run (again)
sudo apt update and post the output. If all is ok, at the end it will state that there are updates (I hope)