If I use CSS Flexbox property then do I need to use CSS Grid?
There is no right answer to your question. You may only need to use flexbox, you may only need to use grid, or you may need to use both. It completely depends on your content and the layout you are trying to implement.
Some areas are best for a Flex container and some for a Grid container. It depends on the situation like stated above. For a navigation bar for example I would always use flex for a gallery of picture I would use grid.