There is no /opt path in the provided images, so you will need to download the luci source yourself, except of course make is not included in the images either, so you will have to build that from source as well, and who knows what else.
None of this would be necisary if there was a repo for openwrt that the espressobin could fetch packages from, but theres not. The architecture directory on the openwrt repos is empty.
One has to ask oneself: did anyone even try any of these instructions before posting them here?
these instructions nowhere mention building LuCI from source on the board itself, they just show how to enable and compile LuCI on a local machine.
About The Package
Quick User Guide
Getting Started Tutorials
Where to Buy
Ports and Interfaces