In the UserGameSettings.ini configuration file, there is a setting called AdminPassword.
The value of which is what you need to use to enable cheats, you can do this by typing enablecheats followed by the password.
E.g. enablecheats password123
Please note that if you change the AdminPassword in the configuration file, it will not change on the server.
To update the AdminPassword, you have to create a new custom command line.
You can also place your steamid64 into the AllowedCheaterSteamIDs.txt file, you can find what your steamid64 number is on this website: https://steamid.io/
Once the steamid64 is in there, just restart your server and you will be able to use admin commands.
Remember admin commands must have cheat or admincheat at the front to work.
E.g. admincheat DestroyWildDinos