Hi! Welcome!
Two answers, depending on why and when you want to hide it:
1. A published deck without a visible menu bar is "Protected/Locked" -- this is done in a different save menu than the usual one because a locked deck is stuck in Interact mode and can no longer be edited. This is usually the final step before publication.
You can find this option in the menu under File > Properties, where you can also adjust the name and author properties of your project if you'd like to and then you can save the locked/protected/"no menu bar" version of the deck with the "Protect..." button.
2. While you're making your deck, and while you have a drawing tool active you can hit the "M" key on your keyboard to hid the menu bar. It's very useful to make sure your cards will look how you want them to when the menu bar isn't visible anymore.
I hope this helps!