Create Formula and Roll-Up Summary Fields

From Setup, click Object Manager, and select Job Application.
Click Fields & Relationships, then click New.
For Data Type, choose Roll-Up Summary, and click Next.
For Field Label, enter Number of Reviews.
Click Next.
Select Reviews from the Summarized Object picklist.
Select Count as the Roll-Up Type.
Select All records should be included in the calculation as the Filter Criteria.
Click Next, Next, then Save & New.

have create it according to this but still getting an error