Operator status is what lets you run commands like /gamemode and /tp. On a brand new server nobody has it, including you, and the only place that can grant the first one is the console.
Why it happens
Operator can only be granted by someone who already has it, or from the server console — which is trusted by definition. That is why the first op always comes from the panel.
What to do
- Start the server and wait for
DoneCommands typed before the server is ready go nowhere.
- Type
op YourUsernamein the consoleNo leading slash: the console does not use one. Use the exact spelling and capitalisation of your Minecraft name.
- Rejoin if you were already connected
Permissions are applied on join, so an existing session may not pick them up until you reconnect.
The console replies Made YourUsername a server operator, and in-game you can run /gamemode creative without a permissions error.
Operator is total control over the server, including commands that can destroy the world. Give it only to people you would trust with the file manager.