you need to change this if you where installing with older ISO…
simply remove the entry from your /etc/pacman.conf and rerun sudo pacman -Syu and you will be up.
Latest released ISO is already using the changed config:
You can find latest pacman.conf here in case:
https://raw.githubusercontent.com/endeavouros-team/EndeavourOS-ISO/main/airootfs/etc/pacman.conf
With this you would replace your old one with the new default:
wget https://raw.githubusercontent.com/endeavouros-team/EndeavourOS-ISO/main/airootfs/etc/pacman.conf && sudo cp pacman.conf /etc/
But only use this if you do not have any custom changes on your pacman.conf (like another custom repo added p.e.)
This was announced in may already here: