It's sometimes difficult when one uses instructions that were created more than 1,5 years ago.
Especially when one hasn't got any experience with OpenWrt.
So, there're (at least) 2 solutions:
-
Instead of a snapshot image, use a stable image (23.05.5 at this time).
With this you can follow the rest of the instructions as with the previous image.
Advantages: A stable release with LuCI already built in. -
A little while ago, opkg was replaced by apk (see here).
You could try to download and install LuCI with the new tool.
I'd go with the first solution, since snapshots might or might not have issues whereas stable releases are much better tested.