How to select name by 2 fields in another 2 tables in mySQL
as correctly mentioned in comments - LEFT JOIN not proper choice in Your case, even if You fix the syntax (JOIN require ON condition)

You need use INNER JOIN:

SELECT 
     CONCAT(s1.firstname,s1.lastname) as full_name 
FROM students s1
INNER JOIN registration r1 ON s1.student_id = r1.student_id 
INNER JOIN courses c1 ON c1.course_code = r1.course_code 
WHERE c1.course_name LIKE '%Structure%'; 


or You need swap tables

SELECT 
     CONCAT(s1.firstname,s1.lastname) as full_name
FROM courses c1 
LEFT JOIN registration r1 ON c1.course_code = r1.course_code
LEFT JOIN students s1 ON s1.student_id = r1.student_id
WHERE c1.course_name LIKE '%Structure%';


as You can see LIKE - must have % from both ends for found what You are looking for