How do I assist a category to a certain page

Hello I haven’t assigned a category to a page in a while. I have been trying for hours with no lucky. I have no idea how I created all my website when I can’t figure this out. I have made a new page and a new category to go with the page. When I make a new post I assign it to a specific category that’s fine but I want the post to show in the page related to the category. There is nowhere at all that lets me select a category to a page it’s really frustrating. Also when I create a page it’s not letting me add flex blocks etc to the page it’s just refusing to be dragged or selected. I have no idea why it’s become so difficult when I remember it being so easy when I done it all before. I really hope you can help me thanks