I am beginner :  Not able to solve Trailhead challange for-Developer Beginner :Process Automation-Automate Basic Business Processes with Process Builder
"
Create a process to update child record when the parent is updated.
You've been given a requirement to keep Contact addresses in sync with the Account they belong to. Use Process Builder to create a new process that updates all child Contact addresses when the address of the Account record is updated. This process:
Can have any name.
Must be activated.
Must update Contact mailing address fields (Street, City, State, Post Code, Country) when the parent Account shipping address field values are updated.
NOTE: You may have to deactivate the validation rule for the Contacts object (created from a previous challenge) in order to complete this challenge."

 What I had tried 
Deleted validation rule on contact  from previous challange.
 Using Process builder created new Process with below details
Object: Account
Criteria:'Update Address' with condidtion

Field: selected shipping street, city, state,postalcode and country
Operator: Ischanged
Type: boolean
value: true

Condition:
Any condition met (OR)

 For Immediate action
Action name: Mailing address change
Records: Account.contacts
Criteria for Updating Records:No criteria—just update the records!

Set new field values for the records you update:

Field: selected Mailing street, city, state,postalcode and country
Type: Reference
value:  selected shipping street, city, state,postalcode and country

 after this saved and activated. So Once I check challange got below error:
"Challenge Not yet complete... here's what's wrong:
An update to an account record failed to update the mailing address fields of all child contact records. Make sure that the process is correct and that it is activated."

So Please help.