I am just carrying out these simple steps. Upon checking it I get this error

Add a custom field to the standard account object that provides a rollup summary of the total expected revenue from all related opportunities.The rollup summary field should be labeled 'Potential Value' and have the Field Name of 'Potential_Value'. The resulting API name should be 'Potential_Value__c'.
The rollup summary should calculate the total expected revenue of all the opportunities related to the account.


ERROR I GET
Challenge not yet complete... here's what's wrong: 
There was an unexpected error in your org which is preventing this assessment check from completing: System.DmlException: Insert failed. First exception on row 0; first error: FIELD_CUSTOM_VALIDATION_EXCEPTION, You cannot close the opp without a reason: []

All- I am taking the trailhead challenge from salesforce for Data Security - Records Access. Here is the challenge

Configure appropriate organization-wide defaults
You’ve been asked to create a custom object for tracking projects. The Project custom object records should only be visible to the owner of the record and users above the owner on the role hierarchy. Create the Project custom object and then create an organization-wide default for the Project object to accommodate for this use case.The custom object must be named 'Project' with a resulting API name of 'Project__c'.

I have done as per what is needed. But I still get this error. Can someone help please?

Challenge not yet complete... here's what's wrong: 
An object with the API name Project__c does not exist

I am just carrying out these simple steps. Upon checking it I get this error

Add a custom field to the standard account object that provides a rollup summary of the total expected revenue from all related opportunities.The rollup summary field should be labeled 'Potential Value' and have the Field Name of 'Potential_Value'. The resulting API name should be 'Potential_Value__c'.
The rollup summary should calculate the total expected revenue of all the opportunities related to the account.


ERROR I GET
Challenge not yet complete... here's what's wrong: 
There was an unexpected error in your org which is preventing this assessment check from completing: System.DmlException: Insert failed. First exception on row 0; first error: FIELD_CUSTOM_VALIDATION_EXCEPTION, You cannot close the opp without a reason: []

All- I am taking the trailhead challenge from salesforce for Data Security - Records Access. Here is the challenge

Configure appropriate organization-wide defaults
You’ve been asked to create a custom object for tracking projects. The Project custom object records should only be visible to the owner of the record and users above the owner on the role hierarchy. Create the Project custom object and then create an organization-wide default for the Project object to accommodate for this use case.The custom object must be named 'Project' with a resulting API name of 'Project__c'.

I have done as per what is needed. But I still get this error. Can someone help please?

Challenge not yet complete... here's what's wrong: 
An object with the API name Project__c does not exist