The Navbar menu button is designed to automatically appear on tablet breakpoint and smaller screen sizes. To verify and adjust this functionality in the Designer:
Select the Navbar or any element within it
Open the Element settings panel and navigate to Navbar settings
Under Menu > Show, you can manually trigger the menu to test its behavior
Use the device options slider in Navbar settings to customize the breakpoints where the menu button appears
Hopefully this helps! If you still need assistance, please reply here so somebody from the community can help.