This is how to enter a steam API key to solve this issue.
- Go to http://steamcommunity.com/dev/apikey
- Login with your steam details
- Enter a domain name that you like. Could be: "node.cubes.host"
- Copy your KEY
- Login on your game panel
- In the server overview, click on the name of your server
- Navigate to Configuration -> Startup Parameters
- Enter your Steam API key in the Steam Web Api Key box. Then save it.
restart your FiveM server.
The issue should now be resolved.