Can’t assign page to category

Hello, when I create a new page I can’t assign it to a specific category as it’s not shown in the sidebar, although the category already created and it exists on page categories.

For example: I want to assign my new page to “New Smart Science” category.
Here is the categories shown at the sidebar when I edit or create a new page:

Although the “New Smart Science” already created in the categories as shown in the screenshot: