in the course (Part 3: Advanced Site Design using the Site Editor) lesson 3 (Introduction to templates) is locked after completing the next lesson it is going to be unlocked that means you have to complete lesson 4 then come back to lesson 3 to complete
https://learn.wordpress.org/lesson/all-about-templates/

you just have to delete the lesson 3 and lesson 5