In the summary view of the landing page asset, check to see if it's based on a free-form or a guided landing page template. If it's a free-form landing page, you can drag and drop elements. If it's guided, it depends on how it was coded, but it should have fixed areas to add or remove content - it's not drag and drop. If it was coded to be marketer-friendly it should have have sliders or toggles to turn on/off the content you want to see.
Here's some docs related:
The first thing that you should know, whether it is a guided template or free form template that you are using for your landing page. If it is a guided template then it's not possible to drag the elements on page and if it's a free from Template then you can easily drag the elements on page.
How you will identify the type of Template :
For that go to Design Studio and select the template that you are working on, it will show you the type of template you are using on landing page.
If it is a guided Template then:
And if it is is a free form Template then: