The field 'Overall_Review_Score__c' either does not exist on the Review__c object or it is not of type Number.

@Kolaboina Sai Ganesh Goud

In the Module below steps are given to create a field.

Create a Formula

Create a custom formula field that calculates the overall score from the Review object.

1. From Setup, click Object Manager, and select Review.
2. Click Fields & Relationships, then click New.
3. Select Formula as the Data Type, and click Next.
4. For Field Label, enter Overall Review Score.
5. For Formula Return Type, choose Number.
6. Click Next and add the formula under Overall Review Score (Number) = 

( VALUE(TEXT(Core_Competencies__c)) + VALUE(TEXT(Experience__c)) + VALUE(TEXT(Leadership_Skills__c)) + IF( Recommend_for_Hire__c , 5, 0) ) / 4

1. Click Check Syntax to verify that there are no errors.
2. Click Next, Next, then Save.