Job_Application__r does not exist. Check spelling.

I have the same problem, even though I have finished the required previous trail. However, it is solved now by: 

1. Go to "Review" object under "Object Manager". 

2. Stay in "Review", create a master-detail field by clicking on "Fields and Relationships", and choose "Job Application" as the related. 

3. Now you are ready to proceed the trailhead requirement to create the advanced formula by copy and paste the formula.