First of all welcome to the wonderful world of EndeavourOS.
Please check the first post where @dalto gave steps on how to convert to a working boot loader which is systemd-boot .
There is a file/script you have to run which I did not include in my post that summarise the steps I did.
The summary of my steps are here → [Tutorial] Convert to systemd-boot - #138 by limotux
I hope this helps. Let us know how it goes.