Solved - my problem + How to REVERT to STOCK firmware !
Problem:
Would be nice If someone could help me.
I got two TL-WR841N V13 (one for a friend, the other one for me) and flashed openwrt snapshot on both, with the help of these: tutorial, openwrt-docs-luci and yt-video.
One is working really nice, but on the other one I just can not install luci.
If I try to install luci on the second device (on the first device luci is working well):
/$ opkg update
Downloading http://downloads.lede-project.org/snapshots/targets/ramips/mt76x8/packages/Packages.gz
Updated list of available packages in /var/opkg-lists/openwrt_core
Downloading http://downloads.lede-project.org/snapshots/targets/ramips/mt76x8/packages/Packages.sig
Signature check passed.
Downloading http://downloads.lede-project.org/snapshots/packages/mipsel_24kc/base/Packages.gz
Updated list of available packages in /var/opkg-lists/openwrt_base
Downloading http://downloads.lede-project.org/snapshots/packages/mipsel_24kc/base/Packages.sig
Signature check passed.
Downloading http://downloads.lede-project.org/snapshots/packages/mipsel_24kc/luci/Packages.gz
Updated list of available packages in /var/opkg-lists/openwrt_luci
Downloading http://downloads.lede-project.org/snapshots/packages/mipsel_24kc/luci/Packages.sig
Signature check passed.
Downloading http://downloads.lede-project.org/snapshots/packages/mipsel_24kc/packages/Packages.gz
Updated list of available packages in /var/opkg-lists/openwrt_packages
Downloading http://downloads.lede-project.org/snapshots/packages/mipsel_24kc/packages/Packages.sig
Signature check passed.
Downloading http://downloads.lede-project.org/snapshots/packages/mipsel_24kc/routing/Packages.gz
Updated list of available packages in /var/opkg-lists/openwrt_routing
Downloading http://downloads.lede-project.org/snapshots/packages/mipsel_24kc/routing/Packages.sig
Signature check passed.
Downloading http://downloads.lede-project.org/snapshots/packages/mipsel_24kc/telephony/Packages.gz
Updated list of available packages in /var/opkg-lists/openwrt_telephony
Downloading http://downloads.lede-project.org/snapshots/packages/mipsel_24kc/telephony/Packages.sig
Signature check passed.
/$ opkg install luci
Unknown package 'luci'.
Collected errors:
* opkg_install_cmd: Cannot install package luci.
How I solved the problem + How to revert to STOCK firmware !
1. I did revert to stock firmware : (use at your own risk)
Flash instruction to revert to Stock firmware (same as flashing openwrt/lede, I only had to delete header of original tp link firmware):
Configure PC with static IP 192.168.0.66/24 and tftp server (this yt-video could help) .
Rename "tp_recovery(TL-WR841N(EU)_V13_STRIPPED).bin" (this is my changed version, never use original firmware, when I did I stuck in boot loop 
hopefully I was able to revert)
to “tp_recovery.bin” and place it in tftp server directory.
Connect PC with one of LAN ports, press the reset button, power up
the router and keep button pressed for around 6-7 seconds, until
device starts downloading the file.
Router will download file from server, write it to flash and reboot.
After it finished flashing, configure PC IP back to automatic.
(changed @braian87b's Lede Flash instruction for my purpose, by the way thanks for your instruction 
)
2. I did flash openwrt/lede again
I used this Flash instruction. I connected over scp with Winscp to 192.168.1.1 with account name "root", I stayed on root folder and started the console. Before I typed in the commands from openwrt-docs-luci. I plugged in my Modem's LAN cable to TL-WR841N WAN for the internet connection.
I think, I only had to stay on root folder, but now I can share how to revert to stock firmware 
I just soft bricked my TL-WR841N V13 three times before I realized that I only had to change a small part
I hope I was able to help you.
Yours sincerely.
Important information:
Guys I do not recommend you to revert to stock firmware, Tp-link TL-WR841N stock firmware is infected with a VPNFilter malware, like all other devices on this list. So if openwrt is working well for you, do NOT revert to stock firmware.