この５月に入ってから多い質問。
指示を読んでないのかな？

Are you using a playground that has implemented the following projects as instructed?

Before you can do this project, you must complete the Build a Data Model for a Recruiting App, Customize the User Interface for a Recruiting App, and Improve Data Quality for a Recruiting App projects. The work you do here builds on the work you complete in those projects.

Are you using a playground that has implemented the following projects as instructed?

Before you can do this project, you must complete the Build a Data Model for a Recruiting App, Customize the User Interface for a Recruiting App, and Improve Data Quality for a Recruiting App projects. The work you do here builds on the work you complete in those projects.

You need to create the object as well as install the package.

Review
https://trailhead.salesforce.com/content/learn/projects/build-a-data-model-for-a-recruiting-app/create-a-custom-object-for-reviews

Job Posting Site
https://trailhead.salesforce.com/content/learn/projects/build-a-data-model-for-a-recruiting-app/custom-object-job-posting-sites

Job Posting
https://trailhead.salesforce.com/content/learn/projects/build-a-data-model-for-a-recruiting-app/junction-object-job-postings