Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE Student_Tests_Taken (registration_id VARCHAR, test_result VARCHAR)
TABLE: CREATE TABLE Student_Course_Enrolment (date_of_completion VARCHAR, registration_id VARCHAR)

### Question ###
What are the completion dates of all the tests that have result "Fail"?

### Accurate SQL ###

SELECT T1.date_of_completion FROM Student_Course_Enrolment AS T1 JOIN Student_Tests_Taken AS T2 ON T1.registration_id = T2.registration_id WHERE T2.test_result = "Fail"