Well, firstly, you want to make sure that you're actually an operator on your server -- in order to set yourself as an operator: go to the Home tab on your Minehut control panel and in the Console tab write "op {your username}". When you are sure that you're an operator, I would recommend that you install the Essentials plugin by going into the Plugins tab on your Minehut control panel and searching for "Essentials", then selecting "Install Plugin" once you've found it. Make sure that you stop and start your server after doing this! Once in game, you can do /gmc to enter creative mode.
Optionally, afterwards, you can go to the Server Properties tab and select the Gamemode dropdown box, from where you can set your default gamemode to Creative.