After installing Source mode you need to add an administrator, follow the instructions below
- Shut down the server
- Click on "Configuration Files"
- For the "admins_simple.ini" file, click on the "Text editor" button
- Then enter the SteamID and rights in this format "STEAM_0:1:58141852" "99:z" (99:z) indicates the highest rights
- You can read more here: https://wiki.alliedmods.net/Adding_Admins_(SourceMod)
- Start the server

