[How to add Admins]

Yes thanks
After some testing, i do see the problem and i did get it to work. By just adding SteamID in the original USER.eco file from my personal Server folder.
Well...
The File is not getting replaced anymore.. in other words, This would work for anyone having Steam connected to there ECO Game.
Alternative, 
Downloading the file from the FTP and only adding steamID64 as explained in your post.
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Tho... if one do not want there ECO Game on Steam,
How do one become Admin when the SteamID64 is not connected?
As i was saying, if i do,  /whoami = i get an SLG_code.

 

Edit : Ok.. Same process as with the SteamID64 . Just Add the given SLG_code.

as displayed when doing /whoami

[How to add Admins]

When i do this,
Then Push Restart. it do not work.

Then i Push "Save settings and restart server"
Then The  User.eco File gets overwritten by this
____________________________

{
    "MaxSlots": "20"
}

____________________________

Other files i edited in Config folder is not Overwritten,

Another thing,
i do not have ECO on Steam, So i only get an SLG Code..by doing /whoami  not the steamID64 code.