Thinkific presents your landing page to a visitor by finding and assembling the various parts of it. It has a set of instructions do this in your site's Theme.
You can change your site's Theme to include the buy button after your course description and before your course instructor profile by doing the following:
Now you'll have a Custom Landing Page that can be applied to all of your courses through the Design tab of your course creator.
Now you can also stop at step 6 above, and add your course button to the course description directly as HTML: