Hi Joyce!
It sounds like you were trying to fully minimize the sidebar within the Storyline 360 player. I'd be happy to help with that!
Thank you for providing the screenshot! I noticed you have a logo set to display in your course. The sidebar will always be the place where a logo is displayed. If you needed to completely remove the sidebar, then I would suggest repositioning the logo somewhere on the slides themselves or on the Slide Master if you needed it to be displayed throughout your entire course.
It would be helpful to take a closer look at your .story file if you have any additional questions. You can share it here in the discussion or privately through a support case!
